The Discworld Companion is an encyclopedia of all things Discworldian, created by Terry Pratchett and Stephen Briggs.
The book compiles a precise (and often quoted directly from the books concerned) definition of words, lives of historical people, geography of places and events that have appeared in at least one Discworld novel, map, diary, non-fiction book and the short stories "Troll Bridge", "Theatre of Cruelty", and "The Sea and Little Fishes".
The third edition was published in 2003, and includes articles about the later books up to Night Watch, as well as the "peripheral" books and the third short story.
All of them also include information that exists on Pratchett's computer, but that he has not yet worked into a novel (for instance, William de Worde is mentioned in the first edition, six years before the publication of The Truth).
At the end of each article is an abbreviation indicating the book(s) in which the word, person, event or place appeared (if there are too many, then no abbreviation is used). Very useful for any Discworld fan, it is also nearly indispensable for readers who have bought one of the quiz books. It also includes an introduction by Stephen Briggs and an interview with Terry Pratchett, both of which have also been "updated" in each edition.
See here for book cover.
Those looking for GURPS Discworld RPG can find it at http://www.mininova.org/tor/458824.