Despite his Alzheimer's diagnosis, Terry Pratchett is still marvelling at the world and still writing. He tells Stephen Moss about his new scientific direction, battling Hollywood executives – and why he doesn't fear death

Terry Pratchett's entourage has taken over a corner of the White Hart hotel in Salisbury, a long-standing haunt of the writer, who has a house just outside the town. There's his PA, Rob Wilkins, whose role has become more demanding since Pratchett announced in 2007 that he had a rare form of early-onset Alzheimer's; a publicist; and Mr Boggis, a fan who describes himself as "PA to the PA". Mr Boggis is named after a character from Pratchett's Discworld series; he says he has read all 39 books. I never do discover his real name.

There will be another Discworld book, Raising Steam, in the autumn, but it is nothing if not an industry: this month, Pratchett publishes volume IV in his Science of Discworld series, in which chapters of fantasy alternate with chapters of surprisingly demanding Roundworld [aka Earth] science, supplied by Pratchett's long-time collaborators, mathematician Professor Ian Stewart and biologist Dr Jack Cohen.

The Pratchettian premise is that while Discworld is an entirely logical entity – "flat, circular, held up by four world-bearing elephants standing firmly on the back of a giant space-faring turtle and inhabited by ordinary humans, wizards, witches, trolls, dwarves, vampires, golems, elves, the tooth fairy and the Hogfather" – Roundworld is a largely incomprehensible and obscure little place orbiting one of 200bn stars in a galaxy that is itself one of 200bn galaxies. What is it for? How was it made? Are we – its inhabitants – alone? Is there a God? These are the conundrums Pratchett and his co-authors explore.

"It's useful to go out of this world and see it from the perspective of another one," says Pratchett. "There's a lot of science in it, and as Slartibartfast [in The Hitchhiker's Guide to the Galaxy] said: 'I am a great fan of science, but I cannot do a quadratic equation.' I've never, ever been able to do one. I remember one occasion at Warwick University, when Jack and Ian were at their wits' end because I couldn't get it. I felt totally ashamed."

So as well as writing the Discworld sections, Pratchett also becomes the everyman figure wrestling with difficult scientific propositions. Is he a frustrated scientist? "I was a very keen reader of science fiction," says Pratchett, "and during the time I was going to libraries, it was good, written by people who knew their science."

By the time he was in his teens, Pratchett was writing science fiction and attending conventions. "The first convention I ever went to, I met Arthur C Clarke, Mike Moorcock, John Brunner, and just about everyone who was around. This was like going to see the immortals, and you suddenly realised that they're science fiction writers and they're human beings and you are a human being, so there's no reason why you couldn't be a science fiction writer."

Pratchett was an only child in a working-class family in Beaconsfield, Buckinghamshire. Born in 1948, he grew up in the fevered 1960s, when the beginnings of space travel coincided with a golden age in science fiction. "My mum was fairly pushy, before pushiness was in vogue, and it was her who got me writing." She was of Irish heritage and loved telling stories – a gift she passed on to her son. He says his life can be summed up in one sentence: "Terry Pratchett, from birth to death a writer."

Despite his mother's pushiness, he underperformed at school. "I couldn't get on with the headmaster, and he couldn't get on with me. He really wanted the 60s not to happen, and of course all us boys were very happy that the 60s were happening." Was Pratchett a natural rebel? "No," he says, "a natural complainer." He is, though, thankful he fell out with the head. It made him leave school at 17 to become a journalist on the Bucks Free Press, the perfect apprenticeship for a would-be writer. "Local journalism is journalism," he says. "If you get it wrong, they know where you live. You see more things and do more things than you would ever see or do on a mainstream newspaper. I saw my first dead body on my first day at work."

He moved from local journalism to a PR job with the Central Electricity Generating Board, but never stopped writing and reading; he is an autodidact who devoured everything that came his way, and says he has three libraries at home. "If the government ever imposes a tax on books – and I wouldn't put it past them – I'm in dead trouble," he says. His early novels did reasonably well, but his breakthrough came in the mid-1980s with Discworld; 2013 marks the 30th anniversary of the first of the series, The Colour of Magic. Much to his relief, he was able to give up the day job and concentrate on writing.

How did he hit on the idea? "It just happened," he says. "It started off as making fun of fantasy fiction. But when The Colour of Magic sold out in hardcover on the first day, I thought I'll do another one." He says the books got better as the series progressed. "I was older and had done more things and thought more things, and just had more experience to put into it." His satire of fantasy books became a satire of the world at large – he says he extended his targets "to stop myself being a man who shouts at the television".

Pratchett still loves attending Discworld conventions, and especially meeting his new generation of readers. "I find those children who have been brought up by two Discworld fans are rather like the Midwich Cuckoos – as bright as buttons." He also likes the fact he has readers of all ages. "Fantasy is uni-age. You can start it in the creche, and it follows you to death."

What unites Discworld readers? "They are serendipitists," says Pratchett, "pickers-up of interesting things you hadn't expected to see. That applies to most science fiction buffs." He says they are also open-minded, and that it wasn't an accident the first interracial kiss on TV happened on Star Trek. "It fits into what science fiction is, which is people being people and not worrying about what shape, size or colour you are. It's hard to read a lot of science fiction and be a bigot."

Pratchett refuses to be bowed down by his illness, and is producing as much as ever, though these days he finds it hard to type: he dictates into a computer and then edits the text. "I'm at an age when all my contemporaries are forgetting where they left their car keys, and I'm still paddling along quite happily," he says. He can be forgetful, and pauses at certain points during the interview while searching for a word or thought, but says his condition remains manageable.

There have been suggestions that at some point his daughter, Rhianna, who writes stories for video games, could take over the Discworld series. "It will be entirely up to her," says Pratchett. "She's doing very well by herself." He is also looking to the survival of his creation on film, and with Rhianna recently set up a production company, Narrativia, to rectify the surprising absence of Discworld movies. "There's always Hollywood interest," he says, "but Hollywood is full to the brim with people who have the ability to say no and only about one person who can say yes. You could die waiting for Hollywood."

Pratchett is protective of the sort of film that might get made: he says he has twice pulled out of deals. "If Discworld is made, it has to be mine, not theirs," he says. "It's all about the money in Hollywood, but how much money does one person need when what I really want is for it to be done properly? It will happen – one day. And with Narrativia protecting my creative interests, I know it will be faithful to my words."

Despite all this talk of protecting the legacy, Pratchett isn't planning on going anywhere just yet. "They'll have to kill me before I go," he laughs. But when his time does come, he will accept it as stoically as his parents, who always insisted on treating death as a fact of life, and not as a taboo. "It's not morbid to talk about death," he insists. "Most people don't worry about death, they worry about a bad death."

Pratchett supports assisted dying, so that the terminally ill can be given the sort of easeful death his parents wanted. "Death is the commonality of mankind," he says. "Everybody dies. Make it as good as you can while you're doing it, but don't worry about it happening – because you can't ultimately do anything about it." In Discworld, Death is an attractive, sympathetic, sometimes comic figure, a far-from-grim reaper – and Pratchett sees no reason to change his view here in Roundworld.

• Pratchett and his co-authors discuss the science of Discworld on 24 April at Conway Hall, London WC1 (details: waterstones.com). Highlights of the event will appear on theguardian.com/books