BOOKS : Ringworld Engineers

Following on from Rignworld, this books expands and improves upon the original, an uncommon occurance that's for sure! The writer left 10 years between books, and I think it did him good, as he used that time to develop new ideas and a worthwile story, rather than just a story to facilitate the ideas or, worse, a new book.
The human and Kzin characters are back, but everyone else is different. We get to the ring world a lot quicker, and discover many more intricacies about it (a lot provided, and acknowldged, over the preceding 10 years by readers of the original book). The pupeteers are even bigger bastards, some of the 'truths' of the first book ahve been turned into 'lies' to accomodate the new universe, and everything is made to fit (quite nicely) with the rest of the author's books.
This time around the characters are even better examined (except maybe the pupeteer) and some nice problems are solved in satisfying ways.
I would definitely recommend reading both these books, the first was quite good, and the second was pretty darn good but requires reading the first to appreciate it...