Bestselling British author Terry Pratchett, creator of the monumentally successful Discworld series, has revealed that he has been diagnosed with a rare form of early onset Alzheimer's disease.

The 59-year-old writer made the announcement Tuesday through a web posting to fans on the website of illustrator Paul Kidby, who has worked on many of Pratchett's Discworld titles.

Terry Pratchett, above, was Britain's biggest selling contemporary author until the massive success of J.K. Rowling's Harry Potter series. Pratchett's books have sold more than 45 million copies around the globe.
(Gareth Cattermole/Getty Images)

"I would have liked to keep this one quiet for a little while, but because of upcoming conventions and, of course, the need to keep my publishers informed, it seems to me unfair to withhold the news," Pratchett wrote in a message titled "An Embuggerance."

"We are taking it fairly philosophically down here and possibly with a mild optimism," he wrote.

The prolific Pratchett is best known for his Discworld books, a comic fantasy series that satirizes everything from Shakespeare and fairy tales to technology and aspects of contemporary culture. Since the publication of 1983's The Colour of Magic, Pratchett has released more than 30 Discworld titles, many of which rocketed up the bestseller charts.

Prior to the massive success of J.K. Rowling and her Harry Potter series, Pratchett was Britain's biggest selling contemporary author, with his books selling more than 45 million copies around the globe.

His writing has also spawned adaptations for theatre, television, comic books, radio dramas and video games.

In his web note, Pratchett vowed to continue working, saying he is completing his next novel — the non-Discworld title Nation — set for release in 2008 as well as other writing projects.

"Frankly, I would prefer it if people kept things cheerful, because I think there's time for at least a few more books yet," he wrote.

"I know it's a very human thing to say 'Is there anything I can do,' but in this case I would only entertain offers from very high-end experts in brain chemistry." 

With files from the Associated Press