Pratchett, author of the popular Discworld series, has sold more than 55 million fantasy and science fiction books and received an OBE for his contributions to British literature, The Daily Mail said.

"I have been diagnosed with a very rare form of early onset Alzheimer's, which lay behind this year's phantom 'stroke.' We are taking it fairly philosophically down here and possibly with a mild optimism," Pratchett said in a statement posted on the official Web site of his Discworld illustrator, Paul Kidby.

Assuring fans, "I am not dead," Pratchett said he planned to "meet most current and, as far as possible, future commitments, but will discuss things with the various organizers."

Perhaps anticipating offers of help from readers, Pratchett suggested he would not particularly welcome it.

"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," he wrote.