Lucy Burroughs answered
As of June 2012, there are 39 books in the Discworld series.
Discworld is a series of humorous, fantasy adventure stories written by English author, Sir Terry Pratchett. The first novel, The Colour of Magic, was published in 1983. Since then, Pratchett has been writing an average of two books a year, with the latest in the series being Snuff (2011). In the 1990s, Terry Pratchett was the UK's best-selling author, but he has since been overtaken by J. K. Rowling with the Harry Potter books...
Pratchett's Discworld series takes its name from the fictional world in which it is set. The Discworld is a flat world that balances on the back of four elephants, which, in turn, balance on the back of a turtle. The world is inhabited by an array of fantasy creatures, including (but not limited to) golems, gnomes, wizards and trolls.
The Discworld Series: Novels, Short Stories, Science Books and Other Publications
There are currently 39 novels, 6 short stories, 3 science books, and over a dozen other publications - including maps and a cookbook - in the Discworld series.
The Future of Discworld
Pratchett has hinted at the possibility of writing an additional three novels in the series, but he was diagnosed with early-onset Alzheimer's in 2007, and now writes with the help of an assistant or using speech recognition software.