Terry Pratchett / Stephen Briggs - The Discworld Companion
A book of facts about a fictive world. Some parts are lifted from the stories, many are new. And they're all funny.
If you would expect a 'compendium' to be a dry summary of facts, well, you're damn right about the facts part. But even facts are funny on the Discworld. This is good stuff, and a must have for avid Discworld fans.
Note: there seem to be multiple editions, and more than one version of the first edition. Make sure you pick up the right one. (See Wikipedia.)