Terry Pratchett fans get in here!

[Thaluikhain]

I'd also recommend the City Watch books, in particular "Night Watch".

The earlier books, IMHO, weren't so great...you know when an author assumes he is funny and the audience will put up with stupid stuff because of it? There was alot of that. He got better earlier on, but he does tend to pontificate and try to impress the reader with how clever he is, instead of constructing a story at times.

 

[Irridium]

Well, if you have Oblivion on the PC, I suggest Companion Vilja [http://www.tesnexus.com/downloads/file.php?id=28977]. She's a companion that has had quite a few lines of dialog written by Terry Pratchett. Most of it is Thieve's Guild related, but there are quite a few other pieces written by him.

No, its not a book, but I thought I'd share.

 

[Chronologist]

The Color of Magic and Light Fantastic are good ways to start, definitely. Small Gods is excellent as well.

I really enjoyed Moving Pictures. It illustrates very well a key aspect of Discworld - how ideas from our reality (in this case Hollywood) flow into theirs. It's also very well written, with Victor being one of my top 3 Discworld protagonists of all time. If you like Rincewind but are looking for a slightly more competent wizard with none of the magic but quite a bit more courage, then Victor is your man.