As our largest book to date, the Discworld Roleplaying Game includes everything you need to get started, even a customized distillation of the GURPS rules! Create (relatively) advanced technology, rely upon the luck of the Lady, or perform impossible "mad medicine" surgeries.
Command the forces of magic, with rules for playing a wizard, witch, or one of the less-known practitioners such as a demonologist, hedge mage, or even a sourcerer.
Though they came back as a perk, because the answer to that question might include Cohen the Barbarian and Granny Weatherwax, and I don't care to argue with them. And, of course, I got to add one biggie (alongside the much-loved but relatively compact Shouting at Foreigners skill); the new magic system.Which just leaves me having to thank Paul Kidby and Sean Murray for the art, which shows me what things I wrote about actually look like, and Sean for the editing.Oh, and everyone who buys it and says nice things about it, again, without whom the whole exercise would feel really empty.I think it was much better done then the first one or any of the other Powered by GURPS offerings I have bought. And the dueling system for magic is the best I have seen for GURPS. I like to think that we did a decent job of integrating the games rules into the Hellboy text, back in the noughties, which in fact was what made me think "Hey, we could do a Discworld second edition better!" -- but it's a non- trivial task to write the rules into the text rather than appending them.The best answer (apart from, I may hope, "a quick education in the weird charms of the Discworld as a story setting") is probably "an extensive worked example of how to fit the GURPS rules to a specific world" -- very specifically, a somewhat unusual but not totally out-there fantasy setting.