Humourous science fiction is one of the most difficult arts to master, and Terry Pratchett began his career with a couple of pseudo-sf books, The Carpet People and Strata, that were ten years apart and had another novel, the somewhat darkly humourous sf, The Dark Side of the Sun, in the middle of it. It was Strata that first caught my attention, although I rather enjoyed the humour and panache of The Carpet People, since that had some very interesting similarities in a rather unique manner to Larry Niven's Ringworld. Then, in 1983, Pratchett had The Colour of Magic published, and thus was born the Discworld series.
Terry Pratchett writes superb, witty, downright slapstick humourous fantasy, but unlike authors such as Douglas Adams and Piers Anthony, Pratchett doesn't cloak it in puns and churn out endless variations of the same theme. While he has a strange sense of humour at times, Pratchett writes on a more adult level than Piers Anthony does, and it shows in the writing and in the characterisation. For American audiences, getting hold of Pratchett's books has been difficult in the past, simply because his works didn't have a distributor in the U.S. until recently. Once you've read a Pratchett, you'll never turn back. You can start with Mort, Guards! Guards!, or Small Gods, three of my favourite Pratchetts, but it's an even better idea to read the Discworld books in order.
The following is a listing of Terry Pratchett's fiction to this point. (It should be noted that this list does not include a lot of the graphic novels, and some of the other materials that are available. Any information on this material would be helpful, and would be gratefully appreciated by the author.)
Terry Pratchett has his own Web page, and a wonderful little niche in L-space it is, too. It has some terrific information, and also holds some links to other Pratchett places around the Web.