Terry Pratchett is sitting in a central London hotel room, looking like a petite, pointy-bearded wizard. He begins haltingly, with a gentle frost around his words. Then, as he warms up with stories about the seamier side of Victorian life (the subject of Dodger, his latest novel for young adults) and his constantly delayed endeavours to write his memoir (working title: A Life In Footnotes), he undergoes an almost physical transformation. By the time he's recalling his entry into science fiction as a boy (via a shop that stocked porn and was run by an old dear), and reflecting on his prolific output in spite of being diagnosed with a rare form of Alzheimer's (a career-spanning set of short stories, A Blink Of The Screen, was released weeks after Dodger), he appears positively youthful: sharp, wry, glinty-eyed, mischievous.
Fantasy makes him tick. He has been imagining richly comic SF universes for the past four decades, mainly in the Discworld series of novels, for which he has created his own lexicon, but also in a host of children's books and collaborations with Neil Gaiman and Stephen Baxter. To date, 50 of them have been bestsellers, some have been adapted for stage and screen, others won him awards including the Carnegie Medal and a readership in excess of 80 million.
Fantasy is what took him to that old dear's shop in Buckinghamshire, aged 12. At that time, fantasy and SF were obscure sub-genres. "Any shops that sold it were in the big cities, but even then, they were found in the same area as porn," he says.
"In High Wycombe, there was a little shed which was a library run by a very nice old lady in a black dress who served you cups of tea, and who had a collection of eye-watering porn. She would have it all behind a pair of beaded curtains.
I would go in there for the fantasy and I'd see that the gentlemen in the raincoats in the upper levels of the shop were somewhat pink. She had masses of wonderful, second-hand British and American SF and fantasy. I was in secondary school, and I remember thinking around all this porn, 'this is a Harry Harrison [SF author] that I've never seen before'. It's the nerdist gene.
"The old lady quite liked me as her 'kosher' customer, so she'd keep stuff on the side for me. One day I was in there by myself, going through the box of books she'd kept for me, when a plain-clothes policeman walked in. He pointed to me with hostility and said, 'what is he doing here?' I will never forget her face. She picked up a copy of Robert A. Heinlein's Stranger In A Strange Land and said, 'evil be to him who evil thinks'. The man grumbled and went."
The anecdote is a teaser from Pratchett's half-written memoir. Most recently, though, he has been focusing on turning history into an alternate sci-fi universe. Dodger, an adventure fantasy inspired by the Charles Dickens character Artful Dodger, sees its central character travelling through Victorian London's sewers and squalid side-streets, encountering not just an underclass trying to survive, but also Henry Mayhew, Disraeli and "Charlie" (Dickens) along the way.
"Dodger is a fantasy based on a reality. This is a historical fantasy, and certainly not a historical novel," he states. His extensive research into the weird and wacky side of the period has even fed into Discworld. "I had been researching old London for a long time because it's also really useful for Discworld. There you also have a very grotty but also very powerful city."
The publication of Dodger is timely, chiming with Dickens' bicentenary. What does he think of the many adaptations of Dickens' novels - Great Expectations being the latest in a long line? "I would not go into battle with this one but I don't see why you need to do it ... Dickens wrote some very good books. I don't see why they should be rewritten," he says.
Five years ago, Pratchett, who has been married to Lyn for 44 years, announced he was suffering from posterior cortical atrophy, an atypical variant of Alzheimer's disease. Since then, he has become an eloquent campaigner, making a Bafta-winning film on assisted dying, and donating substantial sums to Alzheimer's research. "What keeps me going is the fight. My mum was always up for a fight. The fight keeps you alive, fills you up with fire ... It hasn't dropped, the writing. I can't conceive of a time of not having a work in progress."
There is a small tremor to his handshake, and he searches for a word in one prolonged moment of silence, but there is little sinister in this, for a 64-year-old. He concurs. "If you didn't know, you wouldn't know." For now, he says he is largely battling old age. "You start to get creaky. Bits fall off."
He has always had several books on the go, but his diary is bursting at the moment. Perhaps it is the diagnosis that has led him, defiantly, to tackle the workload. This year, he has been remarkably prolific. He has co-written The Long Earth with Baxter, the last, hugely successful Discworld novel, Snuff, came out last year and he is midway through the next one. After that, he wants to write a sequel to Dodger (in which George Cayley, the pre-eminent Victorian engineer, and Charles Darwin, will appear) and finish the autobiography.
Despite his large body of work, an OBE and numerous other accolades, Pratchett's fans have often expressed outrage at the "outcast status" that his genre-writing is designated by the literary establishment. Does he feel stung that he hasn't ever been nominated for the Booker prize?
"No, it doesn't annoy me. People put it on me that I should want it. When I started out, the chance of making a living by writing was nigh-on impossible. Most writers still have other jobs. I thought, 'okay, go in for journalism. See what happens.' So to be able to make a living out of writing is a benison".
It was years of journalism, for the Bucks Free Press, that instilled in him the discipline that still comes in handy. Nowadays, unable to type, he uses the computer program TalkingPoint, which requires him to speak his stories out loud (his assistant later types up the pages). The change in work practice has not thrown him. In fact it is quicker, and perhaps more fluent, than screwing up pages and starting again, he says. He has even taught TalkingPoint the necessary Discworld vocabulary.
"As a journalist, sometimes you're writing your copy against a wall. What is it you are ultimately doing? You're telling a story. How do you tell it? You use your mouth. The way I'm doing it now, I almost conduct it - I sort of wave my hand. It is actually faster and more sensible than typing."
An hour and a half has shot by and many of my questions have remained unanswered, thanks to Pratchett's nimble conversational zig-zagging. "I played you like a harp," he says with a comic glint in his eye. His has been a stealthy act of sabotage - and immensely good fun.
Dodger (Doubleday $49.99 hardback) is out now.