Terry Pratchett is accustomed to having a dedicated following and has never been shy about engaging with his fans. Between book signings and Chinese meals with them he would take off the wizardly black clothes that make him recognisable and disappear to his home in Wiltshire, his writing and his overseeing of the industry his books have spawned. Now he has credibility within a new constituency, which admires him greatly for talking about Alzheimer's. Pratchett learnt he had the disease three months ago and has now donated a million dollars (£500,000) to fund research.

He talks movingly and openly about the process that has started to fold down his life. It's an embuggerance, he wrote when he disclosed it, though he believes that, with care and persistence, he can live fully for many years yet. Every day he has to relearn how to type, but it doesn't stop him: he has just delivered a book and is about to start another.

Pratchett, a direct man, would brush away any suggestion of heroism. The son of a garage mechanic and a secretary from Beaconsfield, Buckinghamshire, he discovered The Wind in the Willows at 10, read his way through his local library and started to write fantasy stories himself. He ducked out of Alevels to become a journalist on the Bucks Free Press, defended the nuclear power industry as a press officer and was able to give up the day job at 29. By then Discworld, the flat planet that hurtles through space on the backs of four giant elephants, which in turn stand on the shell of a vast turtle, was established and his engaging imagination and comic talent had hooked a cult following. He recently published the 36th Discworld book and regularly heads bestselling lists.

Such success does not impress the literary establishment. As one commentator put it, his work is too populist to be good, too good to be populist. Perhaps A. S. Byatt got it right when she equated him with P.G.Wodehouse and Lewis Carroll. Pratchett refuses invitations to the Groucho on principle, but his defensiveness can make him seem chippy. I am not the only journalist to have found him occasionally pedantic, and to have been irritated by his habit of anticipating your next question before you've decided what it is. He can come across as a clever clogs, though in the next breath he will be modest: “I wouldn't ever confess to having anything other than a big bag of tricks.”

For him, the principal tragedy of his condition is that it threatens his writing. At 59 he has been married for almost 40 years to Lyn, who looks after the finances, which now run into many millions. Not that they spend too many of them: he does deals when they go on holiday and his main extravagance is books. They live with lots of cats and indulged chickens that die of old age on the nesting box. Which is just how he would like to go himself.

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