I have the original paperback version of this book. It is an adaptation of Steve Jackson's Generic Universal Role-Playing Game System (GURPS) to Terry Pratchett's Discworld as a Role-Playing environment. A pretty good "GURPS Lite" extract of the basic GURPS rules is included as an appendix (32 pages) at the end of the book, so you technically don't need anything other than pen(cil)s, paper, and ordinary six-sided dice to run and play a Discworld-based Fantasy Role-Playing Game (FRPG).
For a Discworld fan who doesn't want to play role-playing games, this book may still be of interest: it includes informative capsule summaries of most of the major characters in the Discworld novels, and Pratchett-approved insights into the metaphysics of the Discworld universe. It also contains many monochrome drawing by illustrator Paul Kidby.
For a Discworld fan who has never tried role-playing games, but is interested (and has a reasonable number [2 to 6] of similarly-interested freinds), this is an excellent choice. For a hard-core FRPG player, they probably already know about (and have) lots of GURPS stuff - the relavent parts can easily be used with and in a Discworld GURPS game.
While this was explicitly intended to be (and is) an "all-you-need" gamebook, there is now a "supplement" as well: GURPS Discworld Also (a backhanded pun on "Too" instead of "Two"), that is basically more of the same - more Discworld-specific character-information, rules, etc. The supplement is not needed, but is nice to have...