Back in the late 1980s, I read one of his books, didn't find it funny, and wrote him off as an author that just wasn't for me. And I didn't look at his stuff for 15 years or so.
In 2004, I attended Noreascon Four, that year's World Science Fiction Convention. Mr. Pratchett was the Guest of Honor. I didn't meet him or hear him speak, but enough people I like were praising his work that I picked up a couple of books. And they weren't all that funny. In particular, I found his The Colour of Magic and Sourceror to be unpleasant because their lead character, Rincewind, irritates me mightily.
As it turns out, the problem (if you can call it that) was that Pratchett has gotten better over time. His recent books are far, far better (to this reader's taste) than his early stuff, as you can see in this review. And as I finally get to it, THUD! is the best of the Discworld books that I've read.
The Discworld stories center around different protagonists. There's the Rincewind sequence, the Witches of Lancre set, a bunch that deal with the incarnation of Death, and the Watch books. THUD! is a Watch book. I've read only the first and last (to date) of the Watch books, so the transition was quite sudden. If you have not read earlier Discworld books, spoilers will follow this point.
When we first meet Sam Vimes of the Watch, he's a drunken, broken-down man with no life beyond the Watch, and no respect for his role in the Watch. The Watch itself doesn't deserve any respect. It's as if the Keystone Kops were much more corrupt. By the beginning of THUD!, Sam is a teetotalling Duke(!) respected not just in his own city of Ankh-Morpork but throughout the known world. And he's got a wife and kid that he adores. It was quite a shock for this reader (who probably shouldn't have skipped the intervening books).
The story is almost all Sam (although some chapters follow the werewolf Sergeant Angua) as he deals with the problem of multiculturalism in a world of thousands-of-years-old ethnic and racial hostility. No kidding, that's the theme of the book and central conflict. It's humorous fantasy, so it's warfare between the dwarfs and trolls, but the relevance to our own world is not meant to be hard to spot. In the end, the entire thing comes down to Sam himself.
Don't get the idea that it's dead serious. THUD! is hilarious. As a former Blackberry administrator, I liked Sam's interactions with his Gooseberry personal assistant (although it's really more like a Palm device than a Blackberry ...). The dialogue is masterfully witty and funny.
I think what makes this better (again, for this reader) than some of Pratchett's earlier stuff is twofold. One, Vimes is enormously more sympathetic than Rincewind. And two, Pratchett has learned better how to balance the funny parts of the story with the more serious parts.
I strongly recommend THUD!.