Frankowski is a Real Engineer with his own engineering company, who also writes competently; but he reserves his imagination for the details rather than the grand concept. Thus, he uses one of the standard models of sf, going back at least to Mark Twain's Yankee at the court of King Arthur. You take a man with a good understanding of modern technology, drop him into some other era of time or alternate world in which his knowledge is valuable, and lo! — the story writes itself (I exaggerate). This book is a typical example of the breed (The Practice Effect is a less typical example of the same breed).
In this case, the man is dropped into Poland in the year 1231. He's a Polish engineer, so he has no trouble with the language, and all sorts of ways of making himself useful. However, he has a deadline: he knows that in ten years Poland will be invaded by the Mongols, and lots of people will be killed, including perhaps himself, unless he can find a way to change the course of history.
Frankowski is one of those people (Harry Harrison is better known) who delights in redesigning antique machinery, and he takes pleasure in showing exactly how thirteenth century technology could be improved from a standing start. He doesn't go on at inordinate length about it, and there's plenty of human action; but still I wouldn't recommend this sort of book to those of you who don't normally read sf.
If, on the other hand, you've enjoyed this sort of book before, then Frankowski probably deserves your attention; though I should warn that the story's conceived as spreading over four books, and this one doesn't really stand on its own, ending merely at an interlude.
Originally published in Thurb 13 in March 1986