“Of all the dead authors in the world, Terry Pratchett is the most alive,” said John Lloyd at the author’s memorial in 2015. This sentiment remains as true now, 40 years after the publication of Pratchett’s first Discworld novel, The Colour of Magic. The anniversary has been commemorated in a set of illustrated Royal Mail stamps. There’s been a show dedicated to Pratchett at the Edinburgh fringe. A Kickstarter for a graphic novel adaptation of Good Omens, the book he co-wrote with Neil Gaiman, became the number one comics campaign in Kickstarter history, bringing in more than £2.4m; a second series of the TV adaptation was also released.

All this would make 2023 an impressive year by any writer’s standards – but happened in the month of August alone. The big event comes this month, with the publication of A Stroke of the Pen, a collection of rediscovered early stories. This past year also brought an animated adaptation of Pratchett’s children’s novel The Amazing Maurice, and a new set of Discworld audiobooks.

These recordings saw an attempt to drag Pratchett into our contemporary culture wars. The Daily Telegraph reported that A’Tuin, the giant interplanetary turtle that carries the Discworld on its back, had been brought “thudding back down to earth” because of a warning on the audiobooks: “The first book in the Discworld series – The Colour of Magic – was published in 1983. Some elements of the Discworld universe may reflect this.”

Pratchett, in spite of his undeniable culpability when it comes to the fact of being alive in 1983, is an unusual target for this kind of story. After all, this was a writer who first made his name satirising the objectionable excesses of an earlier generation of fantasy writers.

“Terry Pratchett managed to combine all of the best parts of the classic swords and sorcery writers, while stripping out the objectionable relics of their period – the racism and misogyny particularly,” says fantasy author Alex Pheby. “What he put in place of the bad stuff was humour, and a progressive, inclusive treatment of a wide variety of characters. I found that a pretty unbeatable mix, and it’s one that’s really secured his legacy.”

It’s true that Pratchett has escaped the opprobrium often directed towards writers in that difficult period around the end of their careers – but then he was never a critical darling. “He’s a complete amateur,” the poet Tom Paulin famously declared on BBC Two’s Late Review programme when Pratchett was on his 17th novel in the multimillion-selling Discworld series. “He doesn’t even write in chapters.”

Not that Pratchett was universally disparaged: the critics who actually read his books tended to like them. When AS Byatt reviewed Thief of Time, she said it deserved to win the Booker prize. Of course, Pratchett never got a look in. “Thank goodness,” he said, “because I think my earnings would have gone down considerably if I suddenly got literary credibility.” He also joked that when he was given an OBE for services to literature, those services “consisted of refraining from trying to write any”.

But here, for once, he was wrong. It’s the literary quality of his work that has ensured his continuing popularity. “Pratchett’s lines stay with you,” says journalist and fan Helen Lewis. “I first read him as a teenager and, a quarter of a century on, he is still in my brain all the time. Not just his jokes and wordplay, but the way he saw the world – humane, not dogmatic, amused and amusing.”

As Lewis also points out, revisiting the Discworld often has surprising contemporary resonance: “There’s so much in Pratchett’s novels that feels more relevant today than ever. He deals with eugenics and the idea of ‘lesser races’, gun control, sexism, religious fundamentalism, crumbling local government infrastructure … ”

The list goes on. My own most recent Pratchett reread was Making Money, a novel that has often been highlighted as an example of Pratchett’s perspicacity, thanks to its superb depictions of the precarious unreality of banking systems. This subject gained a lot of currency among cultural commentators following the banking crisis in 2008; the difference between Pratchett and all the other analysts was that his ideas were published in 2007 – a year before the crash.

When I read Making Money this summer, I was even more struck by his portrayal of ancient clay golems beginning to develop independent intelligence after ingesting huge quantities of written material. Had Pratchett predicted ChatGPT?

I put this idea to Rob Wilkins, personal assistant to Terry until Alzheimer’s carried him away in 2015 and now the head of the Pratchett literary estate, but he wasn’t convinced. “I don’t think he could have foreseen the way in which artificial intelligence is integrating itself so quickly into our everyday lives.”

I may have got carried away. It’s so easy to get lost in the alternative reality of the Discworld that it feels as though Pratchett is addressing our most immediate concerns. “He is, of course, writing about us,” said Byatt, but that doesn’t necessarily make him a prophet.

“Making Money and Terry’s prediction of the banking crash is always held up as one of the great examples of how he predicted the future,” says Wilkins, “but I actually believe that it had more to do with his ability to read and predict the human condition and our own ghastly ability to continually cock things up.”

That understanding, as much as anything, may help to explain his enduring power. But Wilkins does think there is some insight to be gained from thinking about Pratchett and AI. “Terry’s words and phrases were so beautifully polished,” he says, “that I would like to believe he would immediately see through any computer-generated prose. Even when he had become very unwell, he could still look out through the haze of Alzheimer’s and immediately spot any attempt by me to slip my own words under the wire. Dreaming up a future that seemed out of our grasp wasn’t all that hard for Terry Pratchett. He would, however, remain livid at the continued wait for the elusive hoverboard from Back to the Future.”

• Terry Pratchett’s A Stroke of the Pen will be published by Doubleday on 10 October. To support the Guardian and Observer, order your copy at guardianbookshop.com. Delivery charges may apply.