I just finished reading Thud! and Making Money by Terry Pratchett. They’re both excellent books, and if you haven’t read any of his work you probably ought to. It’s sort of that British-style Douglas-Adams-y thing, but applied to fantasy instead of sci-fi.
Thud! is about a racewar brewing between Trolls and Dwarves and the cops who are trying to prevent it. Making Money is the story of an ex-crook who is put in charge of a bank via inheriting the dog that owns it. They both have plenty of comedy-meets-fantasy-parodies-reality to them. All on a world riding on the back of a giant turtle, of course.
You could always start with The Colour of Magic at the beginning of the series, but you’ll soon reach some of the less-than-awesome books (like Eric). If you don’t start the beginning you’ll miss out on Rincewind the incompetent wizard, Death and his companion The Death of Rats, and some of the other great characters he hasn’t done too much with lately. Most of the books are pretty standalone, so you could start anywhere. If you’re considering reading Thud! I’d recommend Monstrous Regiment first, and for Making Money I’d recommend Going Postal first. They’re not essential, but they develop the background of some of the characters a bit more so you get a bit better depth.
His books make good gifts for mudders and friends of mudders, so it’s something to keep in mind if you’re getting kind of last-minute on your holiday shopping. They’re always in stock at pretty much every bookstore.