I'm with Mav on starting with the whole Discworld series, but start with book 1 and move on from there. Sometimes, he'll refer to something one of the characters did in a previous book so it's always nice to know the histroy.
Once those bazillion books are done, you can move on the the Carpet People books if you like series, but he has a slew of other funny books you can choose from. My whole English-reading family, including me, absolutely adores his writing.
One that I've found many Pratchett lovers are not aware of is "GOOD OMENS. The Nice and Accurate Prophecies of Agnes Nutter, Witch"
It's a collaboration between Terry Pratchett and Neil Gaiman (best known for his "Sandman" comic-book series - which are absolutely terrific, by the way.)
Good Omens is a real treasure in my opinion. I've read it several times.
