The book in question is The Science of Discworld, with prose bits by Terry Pratchett and science bits by Ian Stewart and Jack Cohen. The Discworld, for those of you who don't know and can't bother to look it up, is a flat world that rides through the space on the back of four elephants, which in turn stand on the back of a giant turtle. Magic is more common there than any sort of organized science, and events there are driven by an inherent desire to follow narrative causality. While the Discworld has a cast of magnificent characters, this book deals with the wizards of the Disc's premier school of magic, Unseen University. This includes Rincewind, his Luggage, and the Librarian. Through a series of events involving a magically-charged squash court, faulty heating, and an ant-powered computer a miniature universe is created. Initially put-off by the universe's lack of giant turtles and elephants, as well as its tendency to put everything into balls, the wizards eventually create Roundworld.
Roundworld, for all intents and purposes, is our world. Utilizing magic and special suits, Rincewind (and later the other members of the staff) observe the Roundworld and try like the dickens to get some sort of intelligent life to evolve, and it's this sort of conflict which drives the prose bits of the book. Interestingly enough, Death never makes an appearance, though this might be because no new major characters are introduced and Pratchett didn't want to kill off any of the wizards.
You may be wondering why I kept saying prose bits, and where the science comes in. Well, the way the book is set-up, there's a chapter of prose, and then a chapter of science which relates (sometimes tangentially) to it. These have no formulas in them, and basically deal with how things got to be the way they are. Ah, and space travel. They're pretty good, the science bits, sort of like an exceptionally personable teacher who knows you're only taking their class because it's required.
All in all, a pretty good book, both for the Discworld bits and the science bits. It has Rincewind in it, after all, and he's one of my favorite characters from the series. Only thing about the book is that you have to get it from the United Kingdom, or get it from someone who got it from the United Kingdom, so it'll cost you about $10, plus shipping. It's worth it, though.