This is the third book in Niven's occasional Ringworld series. I first read it in 2000, but reread it recently, so I'll put a brief review here.
It's all quite readable and well written by Niven's standards, but the odd thing about this book is that the first half of it is taken up with preliminaries and irrelevancies; if you want to follow the continuing story of Louis Wu (the main protagonist of the whole series), you can skip Chapters 1–4, 6–8, 10, 12, 14–17. In fact, if you're in a hurry or rereading the book, I think you could simply start reading at Chapter 18 without missing anything essential.
The real story of the book (as opposed to the sideshow in the first half) is about the problem of looking after the Ringworld and saving it from potential disasters. There's already a protector attempting to do the job, but he may not be up to it on his own, and his relations with Louis Wu and his companions are uneasy.
There are various other protectors operating on the Ringworld, but protectors don't usually get along with each other unless they're from the same base species.
(If you don't know what a protector is, you should probably go back and read Niven's Protector before starting on the Ringworld series.)
I find it rather implausible the way the main characters in this story draw up contracts with each other. A contract has value only if it can enforced in some way by both parties. But there's no effective system of law or law enforcement operating on the Ringworld as far as the protagonists are concerned; and how would you go about forcing a protector to honour a contract, if he decides not to?
This book doesn't really make sense as standalone reading. You'll read it only if you want to know what happens between the second and fourth books of the series.
Written in August 2009