For an author whose fantasy creation is the subject of intense Internet discussion, Terry Pratchett, inventor of Discworld, seems strangely unmoved by the electronic revolution. “If you are not careful, the Internet can be quite an expensive way of becoming stupid,” he says.

His eyes do light up at the word “book” - and it’s not only because he has sold six and a half million of them worldwide. “A book is the ultimate interactive entertainment,” he says. “There are 26 letters of the alphabet and a handful of punctuation marks. And the smell and the sight and the sound I can create in your head will be similar to the smell, sight and sound I can create in someone else’s, but it will not be the same because it will be filtered through your own experience.”

The man who was turned on to reading by The Wind in the Willows - his favourite character was the home-loving yet adventurous Mole - becomes animated: “Even more so than radio, which as we know has the best scenery, a book can have the best scenery, the best sound effects, the best special effects.”

Yet some of Pratchett’s work is available on CD-Rom, and his fans not only e-mail the author, they also haunt two forums on the Internet: alt.fan.pratchett - “for general fannish discussion” says Pratchett - and alt.fan.pratchett (books) “for analysis of the motives of characters” and the multitude of arcane references in Discworld.

So he is by no means against the information revolution, just familiar enough with it to be critical. “I get rather weary when some damn computer has a CD-Rom nailed to it and is then called a multimedia extravaganza - though there is good stuff out there,” he says. And, like Mole, he looks forward to change: “I am quite certain that if our civilisation continues books will be replaced by something else. But it will be something in which there is a linear narrative . . . There will still be a story.”

Essentially, that is what is at the heart of Pratchett’s success: telling stories. Whether for children, as in his first book, The Carpet People, written when he was 17 and published when he was 20; for young teens, with the more recent Johnny Maxwell trilogy (adventures taking off into other worlds from contemporary small-town life); or, for teenagers young and old, the baroque splendour of Discworld, a flat planet carried through space by four elephants standing on the back of a 10,000-mile-long turtle.

He doesn’t despise the fantasy label although it has sometimes led to his being marginalised as a genre writer. A fierce debate in the books pages of The Sunday Times followed his appearance alongside Austen, Dickens and Bront in a guide for those who wanted to be well read.

“Sooner or later,” he says, “you have to choose a flag and rally round it and,on that basis, I’d rally round the fantasy flag.”

His reason? “Because it is the basic literature. When guys sat around fires, 10,000 years ago, they couldn’t possibly have told one another stories about the agony of going through the male menopause while being a junior lecturer on some Midlands campus. And what they told one another was fantasy - stories of gods and heroes.”

He does admit, however, that “where this high-flown sentiment breaks down a bit” is that “a lot of what’s called fantasy isn’t actually very good.” Again, the 48-year-old writer who enjoys quoting GK Chesterton takes a judicious view.

The unique selling point of the Discworld novels, which attracts those who never normally read fantasy (whether they be teens or their mothers), is their irony, allied to lashings of broad pantomime humour. Yet Pratchett says, “I do write serious fantasy. Funny and serious aren’t opposites. I couldn’t write solemn fantasy, because I don’t think, in the post-Tolkien world, it’s very easy to do. We know too much about kings. And we know too much about wizards. Or modern wizards, who tend to be scientists and technologists. There’s only so long we will suspend disbelief.”

Though still leavened with much humour, this earnest streak is more easily discerned in the moral dilemmas of the Johnny Maxwell series: Only You Can Save Mankind, Johnny and the Dead and Johnny and the Bomb. Pratchett hates books where fantasy is used “as a handylittle plaster for kids”; in which if a child’s father is dying “he goes into a fantasy world and deals with the situation as a metaphor. It is using fantasy as a little pink pill,” he says.

In Only You Can Save Mankind, the aliens in Johnny Maxwell’s computer game ask him to save them from annihilation. On the verge of adolescence and coping badly with “trying times” between his parents, Johnny nevertheless responds to the challenge. “He takes the view,” says Pratchett, “that it may be real, it may not, but you have to deal with it as if it is.”

Despite his earthy approach to the existential angst of youth, Pratchett snorts at the teachers and librarians who decry fantasy as “not relevant to the child’s experience”. “I should hope not,” he says.

His strategy of grounding the Johnny Maxwell stories in reality may have been sparked by his early favourite, The Wind in the Willows. “Are they animals? Yes. Or are they humans? Well, sort of. They are both animals and humans. Most kids,” thinks Pratchett, “will say ‘fine’.” As he himself did.

Although he grew up in “a landscape written by Richmal Crompton”, an only child exploring a small patch of idyllic countryside with a gang of friends, he relishes writing for the children of today who have been fed a rich diet of fiction in films and on television as well as in books.

In the latest Johnny Maxwell book, Johnny and the Bomb, the children switch back and forth in a complicated sequence between different legs of Pratchett’s “Trousers of Time”. Pratchett knows his readers can handle this. “They’ve seen Blade Runner, Back to the Future, they’ve seen Mad Max. Their grandparents watch Star Trek.

“One objection to fantasy is that it is a world where anything can happen, ” he says. Yet these children have seen so many time machines they know what one should look like by now. “They are constantly analysing the reality around them by reference to a fictional world that they also inhabit.”

This means that he can have fun testing the boundaries and playing with their ideas of time and space travel. In Johnny and the Bomb, he makes the time machine a Tesco supermarket trolley, commandeered by a bag lady and her psychotic cat.

For older Discworld readers, he can also play with the conventions of story, although he accepts that he is working in a tradition as rigid as that of fairy tale. His empire, overpopulated with wizards, trolls, assassins, sapient walking luggage, bad jokes, good jokes, golems and obstreperous footnotes, must run on something more reliable than the volatile octarine energy that makesthe Unseen University’s books of magic rattle their chainsand disturb the orang-utan librarian.

Yet as a recently murdered character comments in the latest Discworld novel, Feet of Clay: “It all seems very badly organised. I wish to make a complaint. I pay my taxes, after all.” Back comes the reply: “I AM DEATH. NOT TAXES. I TURN UP ONLY ONCE.” Death, one of Pratchett’s most popular characters, certainly carries a scythe, but he also rides a horse called Binky, SPEAKS IN CAPITAL LETTERS and has a real live granddaughter called Susan.

That is how, as they say on Discworld, “The Turtle Moves”. And shows no signs of stopping. For Pratchett, “The problem with Discworld is it’s so beguiling. It’s so flexible that without actually destroying it, I can write a semi-serious religious book, which was Small Gods, or a police procedural, which is Feet of Clay, or a parody of Phantom of the Opera, which is Maskerade. ”

Yet, despite having produced a welter of printed words, this bookish man also believes that, “A book in the sense of paper and pages is just a temporary home for a story and it may well take up residence somewhere else.”

Meanwhile, although he has just finished one of his extensive twice-yearly book-signing tours, to coincide with the publication of Feet of Clay, the twentieth Discworld novel has already been delivered to its new residence at Gollancz, and is due to be published in November.

Sadly, Johnny and the Bomb will be, says Pratchett, the last in the Johnny Maxwell series. Johnny Maxwell lives on a round planet and has to obey different rules, such as growing up.

Terry Pratchett’s publishers, DoubledayTransworld and Gollancz, are offering free books to TES readers. Signed copies of Johnny and the Bomb go to the first 15 readers to send a postcard (with name and address) to: TESTerry Pratchett offer, Children’s publicity department, Transworld Publishers Ltd, 61-63 Uxbridge Road, London W5 5SA. There are copies of The Discworld Quiz Book for the first 20 readers who send a postcard to: TESTerry Pratchett offer, E Dare, marketing department, Victor Gollancz Ltd, Wellington House, 125 Strand, London WC2R OBB.