CHRIS TITLEY drops in to a fictional universe that enthrals millions of readers around the world.

FORTY-five minutes before Terry Pratchett is due to meet them, his fans have formed a queue from the first floor of York Borders right down the stairs. Soon they will spill into Davygate. Pratchett, meanwhile, is sitting in a small, windowless room a floor above them in the bookstore, signing copies of his latest bestseller, Thief Of Time, with a bandaged hand.

The injury is a direct result of his mammoth signing sessions. He has been promoting this, his 26th Discworld novel, since May 3. At each bookshop, he signs around 500 copies. That adds up to 12,500 signatures in all. Ouch.

"People have told me that ice packs give instant relief, but are not actually, in the long term, good for your hand," he says, with a hint of resignation.

So why does he put himself through this? His novels fly off the shelves as soon as they arrive: he sells around a million books a year, making the 12,500 figure seem quite insignificant.

"Have you ever heard of an American breakfast cereal called Force?" he asks, by way of reply. "Force was the best-selling breakfast cereal in the country.

"It sold so well, the managing director stopped advertising - because 'everybody knew about Force'.

"I think it's still going..."

He points out that he has never enjoyed great media coverage, before or after his success, and the book tours create their own momentum.

"I have gone out and done lots and lots of bookshops, time after time. That creates a following. That spreads out. Generally it just works."

Then again, Pratchett admits that a ten-day break from the UK tour made no impact on sales here - Thief Of Time shot to number one in the bestseller lists. But there are other advantages of the tour: "It keeps you in touch. And it reminds you, once again, what sleeping in hotels is like."

His fans, like the fantasy genre itself, are stereotyped as slightly nerdy. Pratchett rejects that view.

If there are three-quarters of a million of them in Britain, "probably about 5,000 are your absolutely dedicated, dyed-in-the-wool, buy-the- T-shirt" types, he reckons.

He expects to meet a lot of university students today. "Only certain types of people have two hours to spare on a working day." Night events are different, and bring "a completely different cross-section of humanity".

The sheer popularity of the Discworld series suggests that Pratchett appeals to readers of all types and ages.

A former local newspaper journalist in the West Country, his fiction was first published in the Seventies. In 1983 Discworld was introduced in his novel The Colour Of Magic.

Since then, his intricate, bizarre, sometimes dark and often very funny universe, based around a planet balanced on the back of a giant turtle, has grown and grown. It is a place where Pratchett can demonstrate his remarkable plotting skills, and a world where he can satirise our own. "Some of the books are thrillers. Some of them are romances. Some are police procedurals, as they are now called. It's a full and complex little universe.

"It reflects what we are pleased to call the 'real world'. It's a very neat way of writing about the real world.

"Everything is metaphorical. You can deal with quite complex issues, like racism for example."

One reviewer described Pratchett as the natural successor to PG Wodehouse and more versatile than Douglas Adams, author of the Hitchhiker's Guide To The Galaxy. He carefully distances himself from such comparisons.

"I don't think an author is in a position to say. It's like those people who set out to write the great American novel.

"Anyone who thinks they are going to write the great American novel is a twit, and a pompous twit.

"What they must do is write the best novel they can. If I said I was the natural successor to PG Wodehouse or more versatile than Douglas Adams, I would be an idiot."

Pratchett does come off the fence when talking about why he thinks fantasy is not taken seriously as a literary genre.

"There's a literary establishment and it's London based," says the author, who lives in Wiltshire. "It networks very effectively, but people who write fantasy and science fiction are not the 'right kind of people'.

"What makes it worse is we don't care we're not the right kind of people. We quite enjoy not being the right kind of people. That makes them even more angry."

By the sheer force of his popularity, Pratchett has overcome this disdain. Not only are his books regularly, and favourably, reviewed, they have been turned into stage plays and computer games.

And now a film is in the offing. He has sold the rights to three of his novels to Steven Spielberg's Hollywood studio DreamWorks for £700,000. They will be put on the silver screen using a combination of real-life action and the latest computer animation techniques.

Although clearly delighted, Pratchett plays its significance down. "People think the author becomes a real person if their book is filmed.

"I got phoned up by one guy about the DreamWorks deal who said, 'this is the big stuff for you then?'

"I said, 'hang on, the last 21 books got to number one...'"

He will not be writing the screenplay: that would make no economic sense. "In that time I could have written another book."

And he has just signed a contract to write another three Discworld novels. The Pratchett universe, like our own, continues to expand.

With five minutes to go before the signing session begins, the queue at Borders is out of the door and up to the entrance to Stonegate. No rest for that aching wrist today, then.

Thief Of Time by Terry Pratchett is published by Doubleday, price £16.99