The Discworld Companion
Produced by Stephen Briggs and Terry Pratchett, the Discworld Companion is the definitive A-Z of the Discworld. It includes alphabetical references to almost everything Discworld, and is a humourous read as well as a useful reference book.
A new version is released at regular intervals to keep up with the new Discworld novels, and for those of you interested in other 'reference' works, see the Discworld Mapp, The Streets of Ankh-Morpork and A Tourist Guide to Lancre.
I would like to point out that, in compiling these pages, I have not taken any material from the Discworld Companion - if anything is repeated verbatim, it will be because both I and the Companion have used the same novels as points of reference.