And then there are the spin-offs - the Discworld computer game, the Discworld collectible figurines, the Discworld jewellery and the Unseen University scarf in the colours of the institution as described in the books. "Good quality," Terry Pratchett assures me. "Very warm indeed."
What do you get as a top-selling fantasist? Money, clearly. Also e-mail. Pratchett will find 200 messages posted daily in alt.fan.pratchett [sic] - "My floating, electronic fan club," he calls it. And some adulation: he is on tour for more than a month in any year, seeing off snaking book- signing queues. But what you don't get is literary prizes.
"On Booker night, do I search my soul and wish I was there?" Pratchett cocks his head. "No, I do not. If they phoned me up and told me I was on the shortlist, I would think that something was seriously amiss. I would wonder if I had inadvertently left a swear word in or something." He adopts, at this point, a yokel accent, denoting peasant humility: "The Booker and the Whitbread is for them as wins Bookers and Whitbreads, not for the loikes of oi."
But Pratchett has not always sounded this reasonable. He has referred before now to "literary wankers in London". He once remarked, "Anyone connected with the Sunday Times will always shove a knife in me and jump up and down on the handle." He is fond of repeating a remark he has heard more than once from librarians, who tell him eagerly what a treasure he is for bringing into libraries people who might then go on to develop an interest in real books. It's a story he plays for laughs, but you wonder whether it niggles him.
Still, here, if he really needs it, is endorsement after a fashion. In this, the UK Year of Literature and Writing, Terry Pratchett is honorary chairman of the Society of Authors, voted in, a spokesman for the society insisted, by a jury of his peers. As such, Pratchett joins a distinguished line of fine writers who have held the post, stretching all the way back to Tennyson. "To the untutored eye," he says, proudly and with precise consonants, "I would appear to be the kind of writer the Society of Authors wouldn't allow anywhere near its premises, not even after three strong baths." But we are at this point sitting in the society's offices in Chelsea, where Pratchett was attending committee meetings in preparation for the prize-giving night on 20 June, on which will be awarded the Betty Trask Prize for romantic fiction and eight other literary trophies. He seems to be fitting in fine. "Do I actually hand over the prizes?" he inquires of a man from the society, "or does Laurie Lee?"
Pratchett, a small man, has with him a huge-brimmed black fedora and a set of black leather saddle bags, slung over one shoulder. On the second finger of his right hand he is wearing a lumpy, death's-mask ring - a present in gold from the company that does the Discworld jewellery line. He talks through a perpetual half-smile, as if nearly everything was wry. And, slightly competitive in conversation, he challenges me brightly at one point to name the year of the invention of the fax machine. "1978?" I say, shooting pathetically in the dark and slightly suspecting what was coming. "Forty-six," he says. "1846!" (He later uses the same tone of voice to tell me that there are 53 male Pratchetts extant in the British Isles. Crazy name, crazy guys.)
In a dialogue conducted recently for GQ magazine with Bill Gates, the Microsoft billionaire, Pratchett was moved to meditate on the terms "nerd" and "spod". "Spod", he reckoned, was now pretty strictly the term for someone in near-permanent communication with their computer screen, whereas "nerd", which has more general applications, seemed to be losing its pejorative edge and becoming a more friendly term. With these strictures absolutely in mind, it is probably fair to remark that Pratchett himself is most definitely a nerd, but perhaps also - albeit reluctantly - a bit of a spod on the quiet.
"At the moment," he says, "the Internet is a phantom. It's used by academics and journalists who don't have to pay for their own telephone time. When you get a bill, your enthusiasm for surfing is reduced." Much of his e- mail is from Americans who want him to explain some of his more idiomatic phrases; he thinks the Englishness of his writing holds back his sales there. An Australian audience, meanwhile, has no such problem; Pratchett is a rave with antipodeans. He once appeared on a chat show there, seated on a plastic pumpkin.
Terry Pratchett is 47. He was born working class in Beaconsfield and he left school at 17 and got a job as a news reporter on the Bucks Free Press. By this time he had written a novel. He found a publisher for it when he was 20. Other novels followed, but slowly, written in 400-word bursts in the evenings after work. He used the money from his first book to buy a greenhouse. He used the money from his second book to buy a bigger greenhouse. With the money from his third book, he bought "a greenhouse with automatic, self-opening vents". When the Discworld series took off in the mid-Eighties, he had taken the greenhouse thing about as far as it would go and he started buying other items, such as a large house in Salisbury and a Jeep. "Also I bought time," he says. "Forty or 50 hours per week of time. That was the best thing."
By that point, after 15 years of journalism, he had moved on to become a press officer for what was then known as the Central Electricity Generating Board, south-western region. "It was," he says, "another job which involved shuffling words around and putting them in a different order." His responsibilities included the presentation of a press-friendly face on behalf of four nuclear power stations. His tenure coincided with an accident involving a trainload of nuclear waste. It was a very slight incident, involving, somehow, the partial derailment of a stationary train. But Pratchett still recalls the phone call he received at home on that night, from the manager of the power station, who opened by saying: "It's not as bad as it sounds."
Discworld took him away from all this. It's a flat planet, borne on the backs of four elephants which are themselves astride a turtle. Characters will be called such names as Glod, Dibbler and Ridcully, though there's occasionally a Susan. Otherwise, gritty realism tends not to enter. Most of the humour derives from crashing the fantasy form's pomposity. Death will take his apprentice, Mort, aloft on a charger and say, "I don't know about you but I could murder a curry." Science students love it. So do schoolchildren. Pratchett has suggested that the books encourage people to question the occult, though this seems a little fancy. Much more they seem to be about encouraging people to laugh. And your ability to laugh will depend on whether or not you find sentences like this funny: "The Horn of Furgle sounded itself when danger was near and also in the presence, for some reason, of horseradish."
Pratchett is not given to trenchant utterances about his craft. "You know the shape, but not the detail," he says impressively at one point. But then, after a moment, he adds: "Sometimes I know the detail but not the shape." So that clears that up.
We do know, though, that he writes in a studio which is "spartan - chiefly because I didn't move in long ago and I've never got round to unpacking most of my clutter". (This includes, I take it, the Sword of Death, given to him by a theatre club following a stage production of one of his tales, which used to hang, illuminated, so as to glow in the dark, in the study in Pratchett's old cottage.) When I called him last week, he answered the phone there, and in the background as he spoke could be heard the music that traditionally accompanies the Can-Can. He assured me this was Classic FM rather than his own selection. Then he crossed to where his CDs were. "Let's see, what's here ... there's Jon and Vangelis ... They Might Be Giants ... Steve Earle and the Dukes, that's terrific ... Genesis ... Meatloaf ... another Meatloaf ... another Meatloaf ..."
The question for fans and curious observers alike is, can he keep it up? He is said to have phoned his editor at Gollancz publishers last year and told him he was thinking of taking it easy, spending some time on other things, and so would turn in only one book that year, instead of the usual two. This was completely fine by the editor. Six weeks later, Pratchett rang again. He'd been thinking about it and, well, actually he probably had got two books in him for the year - so when would it be convenient for him to deliver them?
I ask Terry Pratchett if he ever has any dark nights of the soul, any doubts about freezing up as a writer, the gift just leaving him, the fount of his talent suddenly refusing to flow, the seemingly ceaseless machines of his creativity suddenly seizing and going dry in a terrible grinding of gears after which there would only be silence and the horror of the void. "No," he says.