Terry Pratchett, 59, who has an early form of the illness, said he had come to grips with the news "philosophically" and "possibly with a mild optimism".
Terry Pratchett, who was made an OBE in 1998 for services to British literature, has sold more than 55 million Discworld books worldwide and his work has been translated into 27 languages. Discworld is set in a parallel universe to this one.
Terry Pratchett also said in a letter: "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."
Terry Pratchett said that he is carrying on writing and feels there will be some more books on the way.
