Have you read any Prachett before? If not, I'd try something like Thief of Time or maybe Going Postal before jumping into his latest. You have to kind of get the feel for his work and learn some of the characters before the newest books make sense. (Of course, don't expect to make any sense out of any of his books right away. It's that kind of insanity. I love it.)

Oh! You might try The Wee Free Men as a first Pratchett experience. It's a good representation of his work without being a part of the main series. It would help you get your bearings, I think.