Terry Pratchett was a nice chap with a talent to entertain, who wrote books that are easy and pleasant to read, and funny in places. So I've read most of them, although they vary in quality as you can see from my ratings below.
For a writer of comic fantasy, he had quite strong views on ethics and politics, but he never seemed to put them together into a coherent ideology. Thus, although he had libertarian leanings and distrusted government, he seemed to believe that some form of government is necessary. As to which form of government, he ducked the question by giving his city of Ankh-Morpork a relatively benevolent dictator. This is convenient, of course, but it solves nothing: what happens when the man dies?
A quick introduction to the Discworld
Most of Pratchett's books are set on the Discworld, an improbable flat world operating by magic, where anything can happen, and frequently does. Many stories involve the medieval city-state of Ankh-Morpork.
The Discworld is populated by a rich set of characters, falling into various different groups. These include the City Watch (the police force of Ankh-Morpork), the wizards of Unseen University, the witches who live in the countryside, Death (the Grim Reaper) and his associates, Cohen the Barbarian and his associates, the History Monks... Sometimes a story may involve just one group of characters, but quite often they intermingle to some extent.
The main selling point of the early books was comedy, but the later books become gradually more serious, particularly the City Watch books, which tend to involve crime and political events.
For more background information, see Wikipedia.
Discworld novels
I was initially sceptical about the Discworld books, but eventually started reading them in 1990. I've added my own ratings below, when I can remember enough about the book to rate it. Some of them are good in parts, so they get a mixed rating.
The colour of magic 
.
On rereading this, I found it not quite as bad a book as I remembered.
It's divided into four distinct parts, the first of which is quite good, really.
The other three show considerable imagination, but there's a lack of any overall plot,
and the reader may wonder at the end if his journey was really necessary.
To say that the ending leaves matters unresolved is rather an understatement.
First appearances of Rincewind the incompetent wizard, Twoflower the tourist, the Luggage, and Death.
This book has been filmed and is available on DVD: quite well done, but not essential viewing.
The light fantastic 
. Like The colour of magic,
this has a very slender underlying plot but it mainly consists of Rincewind and Twoflower
undergoing a series of pointless adventures, followed around and sometimes assisted by the Luggage.
It's readable enough but not really what I think of as a novel.
First appearances of the Librarian and Cohen the Barbarian.
Equal rites 
/.
First appearance of Granny Weatherwax, who finds herself trying to look after a young female wizard.
I'm fond of the first half, which is funny, imaginative, and rather endearing.
The second half is not bad, but Pratchett allows the magic to get powerful, scary, and rather out of control, which I think was not a good idea.
Nevertheless, this is a much better book than the preceding two, and I think of it as the first 'real' Discworld book:
from here on there's some continuity in style.
Mort . I think the Discworld's Death character is fine in small doses.
This tells you more about Death than you may have wanted to know,
but it makes a good story if you like that sort of thing.
Sourcery . It's not really bad, but I've never been keen on this one.
Wyrd sisters . The three witches join forces against an evil king,
while the Discworld sees the rise of a superb actor and a dwarf playwright. First appearances of Nanny Ogg and Magrat Garlick.
Leonard of Quirm is mentioned briefly, but doesn't appear.
The first thoroughly good Discworld book, and still one of his best.
Pyramids 
.
I love the beginning in the Assassins' School, and I like Pteppic and Ptraci (who comes on later),
but the actual pyramid stuff is a bit boring and the Ephebian philosophers are very boring
(fortunately they're a small part of the book).
First and last appearances of Pteppic, Ptraci, Chidder, etc.
Guards! Guards! . The first City Watch book,
most notable for the first appearances of Samuel Vimes, Fred & Nobby, Carrot, and Sybil.
The city is terrorized by an enormous dragon, and Lord Vetinari spends some time in his own dungeon.
Eric .
This is a short book about Rincewind,
its main virtue being that it's shorter than the other Rincewind books.
Well, OK, it's quite fluently written and it has humorous moments, it's amiable enough,
but as soon as you put it down you realize you've forgotten what it was about.
Moving pictures . I'm not very keen on this one,
though at least it features the first appearances of Gaspode the dog and Mustrum Ridcully,
the Archchancellor of Unseen University.
Reaper man .
In which Death gets his own lifetimer and experiences mortality for a while.
The Auditors appear for the first time, and the Death of Rats comes into existence.
The main thread of the story is OK, but the whole subplot about the wizards, the undead,
and the supermarket trolleys strikes me as rather pointless and unsuccessful.
Witches abroad .
In which Granny Weatherwax has to fight her evil sister, and Greebo the cat briefly becomes human.
The trouble with this one is that Pratchett is trying too hard to make points and lecture,
whereas I just want to read a good story and be entertained. However, it has its moments.
First appearance of Casanunda (briefly mentioned in Reaper man).
Small gods .
Quite interesting and possibly rather profound, though (a) it's about religion (not my favourite subject),
and (b) it's rather an oddity in the Discworld series, not using any of the regular characters except Death.
I rather like it, but not to the extent of granting it three stars.
Limited use of humour and only the magic associated with gods.
Lords and ladies .
In which King Verence of Lancre is set to marry Magrat,
Ridcully meets Granny Weatherwax for the first time since they were both young,
and Casanunda is trying to seduce Nanny Ogg;
but there's a very serious invasion of evil elves that has to be stopped somehow.
The mixture of comedy and horror is bizarre, but fairly successful.
First appearance of Agnes Nitt.
Men at arms .
An exciting City Watch story with good use made of various characters,
though I'm not very keen on the villain (I'm not usually keen on Pratchett's villains).
First appearances of Angua the female werewolf, Leonard of Quirm, and Foul Ole Ron.
Soul music .
The most significant thing here is the first appearance of the wonderful Susan,
Death's granddaughter, although I feel that better use could have been made of her.
For this book only, we meet Imp y Celyn (translation: Bud y Holly),
the Discworld's first and perhaps only successful rock musician.
You can find here the exotic restaurant menu featuring "Curry with Sweat, and Sore Balls of Pig".
I'm not really keen on the overall story, which becomes too cosmic;
I think Pratchett is best when he keeps his feet on the ground.
But it starts well and has some good ingredients.
In addition to Susan, there's the first appearance of Foul Ole Ron's under-the-bridge gang.
Interesting times .
This is a better-than-usual Rincewind book, because it's not all about Rincewind,
it has a kind of story to it, and it's quite amusing in places.
It's the only Discworld book set mostly on the Counterweight Continent,
occupied by the oriental-style Agatean Empire. Also involves Cohen the Barbarian,
with some brief scenes of the wizards in Ankh-Morpork.
Maskerade . The witches encounter the phantom of the opera.
Quite an amiable read, lacking in particularly bad or particularly good features.
The witches are the only regular characters present. And Death, of course.
Feet of clay .
A City Watch book, in which someone is gradually poisoning Lord Vetinari and a mad golem is at large.
It's a good story, though not one of his best.
First appearances of Cheery Littlebottom and golems (mentioned briefly in Interesting times).
Hogfather .
Most of this book is pretty good; it has plenty of Susan, with side helpings of Ridcully;
Death experiments with an alternative career, and shows his more likeable side.
There are various new and different baddies.
This is also the book in which a restaurant is obliged to make haute cuisine out of mud and old boots.
But the story seems to go astray towards the end, I think because Pratchett starts trying to
say Meaningful Things about Life, which means that he takes his eye off the ball.
This book has been successfully filmed and is available on DVD: recommended.
Jingo .
This is a sort of City Watch book, but one in which Ankh-Morpork attempts to go to war with Klatch.
I previously read it nine years earlier, and it's a better book than I remembered,
though primarily because of the amusing details and the use of characters.
The story makes some sense but it's still a bit of a mess,
and I dislike being preached at even when I agree with the message (in this case anti-war).
The last continent .
Although just about readable if you're short of occupation,
this is a remarkably tedious book by Pratchett's standards,
mostly about Rincewind, the wizards, and the Discworld's version of Australia.
I don't dislike Rincewind, but he's inherently rather dull,
and his habit of fleeing from one predicament to another tends to lead to
aimless, plotless stories that Pratchett seems to make up as he goes along.
Carpe jugulum . Lancre is invaded by vampires.
The witches try to fight them off, and come very close to losing.
The story is exciting and has some funny moments, but overall it's too serious for my taste.
First appearance of the Nac Mac Feegle.
The fifth elephant .
A kind of City Watch book, in that all the City Watch characters are involved,
but in this one Sam Vimes is sent to Uberwald as an ambassador,
so there's a heavy involvement of dwarfs and werewolves, with a light sprinkling of vampires.
Rather unexpectedly, Gaspode the talking mongrel also turns up again.
I think this is actually one of the better Discworld books,
but I can't quite bring myself to give it three stars:
the story is initially funny and later exciting, but it becomes rather disagreeable by Discworld standards,
and I'm not sure that I entirely approve of this.
Incidentally, Sybil reveals that she's pregnant, and the City Watch acquires an Igor.
The truth . In which printing and newspapers come to the Discworld.
Near the beginning, I laughed out loud several times, but it gets more serious as it goes on,
which I think is rather a pity.
Quite a good variety of characters, and I like Otto the vampire photographer.
Mr Tulip, a thug with a talent for art appreciation, strikes me as rather implausible,
but perhaps that's a strange comment to make about a Discworld character.
Thief of time .
This book is good in parts, but it's one in which Pratchett
goes all cosmic on us; and that never works well.
On the bright side, we're introduced to Lu-Tze the ancient sage,
who follows the Way of Mrs Cosmopilite; we see more of Susan (always good);
and there's quite an amusing Igor.
But the Fifth Horseman of the Apocalypse seems like an idea that needs more work;
the abbot reincarnated as a baby is rather tiresome; the mad clockmaker lacks appeal;
and we have a horde of Auditors hanging around (I don't like the Auditors).
The last hero. I haven't read this.
Night watch .
In which Samuel Vimes is thrown back in time about thirty years, to the darker days of his own youth,
when Ankh-Morpork was ruled by a paranoid tyrant and was about to rebel.
Although there are occasional brief moments of levity, this is possibly Pratchett's most serious book
(Wintersmith is also pretty serious), and it works very well.
It's an excellent book with an absorbing story, hard to put down;
and incidentally it won the Prometheus Award in 2003,
which must have come as a bit of a surprise.
Monstrous regiment .
A readable book with some good details and a few laughs, but he never really decided what he wanted to do
with the story, which wanders around rather aimlessly towards the end.
A perfectionist would have called this an intermediate draft and carried on working on it,
but it seems he was more interested in moving on to the next book.
Going postal . The first Moist von Lipwig book,
in which he's put in charge of the Ankh-Morpork Post Office. A fairly compulsive read, with good story and characters,
although it's light on both humour and magic. There are some humorous lines of dialogue.
This book has been filmed and is available on DVD/BD; it makes a good film, and the actors are great,
but the story has been abbreviated and seems much more lightweight than in the book.
Thud! . A City Watch book, with the first appearance of Sally,
her interaction with Angua, and new background information about dwarfs and trolls.
Making money . The second Moist von Lipwig book,
in which he's transferred against his will from the Post Office to the Royal Mint and Bank of Ankh-Morpork,
which he proceeds to turn upside-down in various ways.
Neither the story nor the characters are particularly memorable by Pratchett's standards,
but the book is witty and entertaining and perhaps even mildly educational,
and ends not merely with a flourish but with a series of flourishes.
The chief baddy (Cosmo Lavish) is rather tiresome.
Unseen Academicals .
In which the wizards find an unexpected financial reason to take up the game of football,
an orc proves his worth, and we're treated to the love story of Trevor and Juliet.
This is not one of the best Discworld stories, and the football content doesn't really interest me,
but much of it is entertainingly written and there are some good new characters.
Snuff .
In which Pratchett spends much of the book insisting that goblins are people and should be treated accordingly,
as he has done with various other species in past Discworld books; the message is becoming overfamiliar.
However, the story is well written and quite entertaining, there are a few chuckles in it, and overall it makes a good read.
Interesting to find that orcs and goblins seem to be different species on the Discworld,
whereas they seemed to be different names for the same species in Middle Earth.
Raising steam .
This makes a pleasant read, although it's a story in which not very much happens.
I don't have my own copy.
The shepherd's crown. I haven't read this.
Other adult novels
The dark side of the sun. I don't think I've ever read this.
Strata .
I bought and read this when Pratchett was still completely unknown (pre-Discworld),
and thought it a minor oddity. Not bad; but I didn't go out looking for more Pratchett at the time.
In fact I either lost or got rid of my copy of the book.
Good omens .
On first reading, I found this gripping and hard to put down,
but I emerged from it feeling drained and unconvinced of the value of the experience.
I went back to it only after enjoying Neil Gaiman's on-screen version in 2019, after which I liked the book better.
Nation .
This is a very odd book about young people struggling to recover from a disaster on a remote island.
It could perhaps be regarded as a juvenile novel written for adults (like The lord of the flies).
On first reading, I thought it perhaps the best thing he's done,
and yet for a juvenile novel it's strangely adult in some respects,
and for an adult novel it's strangely juvenile in other respects.
I haven't yet read it a second time.
The Long Earth .
Co-written with Stephen Baxter, there is some flavour of Pratchett here, but it seems diluted.
Neither the story nor the characters are strong;
the scenario is quite interesting though it has some implausible aspects.
The story has no proper ending but just stops temporarily at the last page,
waiting to be continued in the sequel.
Despite all this, I mildly enjoyed it on first reading.
Juvenile Discworld novels
The amazing Maurice and his educated rodents .
Slightly reminiscent of the nome trilogy (further down this page) but somewhat darker.
The wee free men .
We meet the nine-year-old Tiffany Aching, a witch in the making, who temporarily becomes kelda
to a clan of Nac Mac Feegles. All very well so far, but later on she has to rescue
her little brother from the queen of the fairies, and the story turns into an exercise in surrealism,
too weird for my liking.
A hat full of sky . In which Tiffany, now 11,
is apprenticed to an older witch, meets some other young witches, and is possessed by a hiver
(which is usually fatal).
There are moments when the weirdness threatens to get out of control, but overall I think
this is the best and most rereadable of the Tiffany stories.
Wintersmith . In which Tiffany, now 13,
is pursued by the Wintersmith (an elemental force, the bringer of winter),
who has fallen in love with her after she unwisely joined in a special dance.
This is a well-crafted and mainly serious fantasy novel with humorous moments,
but there's a coldness about it in more ways than one.
Tiffany's adventures in each book are so frightening that you wonder how she can emerge sane
from all this.
Other juvenile novels
The carpet people . I don't have my own copy.
Truckers . The first of the nome trilogy (otherwise known as the bromeliad trilogy).
This is a great book, with a coherent plot, distinct characters, and some lovely humorous touches.
Pratchett makes a genuine effort to imagine what it would be like to live four inches high in a world of humans.
I also like the fact that the whole trilogy is sf rather than fantasy, for a change.
Diggers . This is the filler section between the two ends of the trilogy.
Not as good as the other two, but worth reading in context.
Wings . A good finish to the story of the nomes,
in which they hijack a Concorde airliner, meet Grandson Richard, and realize their destiny.
Only you can save mankind. I haven't read this.
Johnny and the dead .
A minor book, but better than you might expect. I don't have my own copy.
Johnny and the bomb (1996). I haven't read this.
Dodger .
A non-magical fantasy about life in early-Victorian London, in which the scenario is grittily realistic
but the characters might be more at home in a fairytale.
Vaguely similar to the City Watch books, but without the Discworld ingredients.
Other things
There are various Discworld spin-off productions and other oddments, often produced by other writers or in collaboration with other writers. Some of them have merit but I don't feel motivated to talk about them here.