(My boss asked me for some Terry Pratchett book recommendations today. It took a bit of research, so I figured I’d share…)
…or “Where to start?” There are something like 35 Discworld novels spanning almost 25 years, so there’s a bit of variation. Pratchett also loves his in-jokes and walk-ons, so you’ll definitely get more out of the books once you’re familiar with the milieu. (You’ll also get more out of them if you know lots of movie, music, and book trivia. The guy’s encyclopedic with his puns.)
The very first book you’ll want to read is Good Omens, written with Neil Gaiman. It’s not a Discworld book, but it has the same kind of style so it’s a good indicator of whether you’ll like Pratchett or not.
He tends to come back to certain sets of characters, so there are “series” of a sort. (Luckily, they aren’t the plodding-along-with-foreshadowing kind of series.) The one I’d recommend starting with is the Watch series:
If that’s a bit daunting, there are others that stand on their own:
Note that I’ve linked to Mysterious Galaxy for these; they’re a great source for backlist Pratchett titles, which are ordinarily difficult to find from any one source.