AS Byatt's journey to Discworld Also, people who don't read, read Pratchett. Boys of 12 who hate books. I hope he is never taught in schools - his biography on the back of the books used once to claim that "some people had accused him of literature," and of course he is literature, but best enjoyed in solitude and retreat. J R R Tolkien used the term "secondary worlds" to describe fictive, invented worlds with their own creatures, geography, history, people. Human beings have always needed the existence of the other, the unreal - imaginary people and things that are other than ourselves - from fairy tales to myths to urban legends. A maker of secondary worlds needs great resources of inventiveness - both on the large scale and in the fine detail. Pratchett's world is wonderful because he has the energy of the great storyteller: you think you know all it is possible to know about a dragon, or a policeman, or a plot or a landscape, and he tells you more, a lot more than you had any right to expect, and this is exhilarating. From book to book he gets better and his world gets more intricate. He gets more and more attached to his own characters, who become more complicated - consider the way in which Captain Vimes grows from being a drunk in charge of a dysfunctional Night Watch to a commander who arrests two armies for a breach of the peace. He finds it hard to go on disliking characters. He can invent irritating minor forms of life: an imp that operates a Gooseberry - or Disorganizer - belonging to Vimes, which is redeemed by the discovery that it can do the office accounts; an accountant called A E Pessimal, sent to inspect the Watchmen, who turns out to be a hero. (Wikipedia constantly illuminates Pratchett. I didn't know that the word "pessimal" means "bad to a maximal extent" or "most wanting in quality or value.") But he can do real evil too: take Mr Pin, the villain in The Truth, or the Chief Exquisitor, Vorbis, in Small Gods - both with the ferocious single-mindedness, true cruelty, and narrow vision which can't change. As Tolkien says, secondary worlds must be coherent. There is a risk of the creator being romantic, or being seen to have designs - didactic or sentimental - on the reader. I reread Tolkien for the landscape and the persistent sense of danger. I have problems with stories of real children who find themselves in secondary worlds - rather as though their reading had engulfed them. J K Rowling is a brilliant inventor of details of magic, but her world has its origin in a boarding school, a place to which I do not want to return. I never enjoyed C S Lewis: I felt he was morally manipulating me as well as his characters. Philip Pullman writes beautifully and dramatically but he is writing against Lewis, and again runs the risk of becoming didactic and controlling. Pratchett, despite the slapstick, the terrible jokes and the very clever complicated jokes, is somehow wise and grown up. As a reader I trust him. I was once asked by a television interviewer, "Isn't all this simply really about us?" and I indignantly replied, "No," because I needed my secondary world to be other, separate and coherent. But he is, of course, writing about us. He is good at policemen, businessmen, fraudsters, murderers, banks and shares, and at music with rocks in it besides, as well as at goblins, witches, dragons, trolls and dwarfs. And of course, computers. But he writes neither satire nor allegory. What gets into his world is in his world, with its own energy and logic. This is an edited extract from the foreword to Terry Pratchett's A Blink of the Screen: Collected Shorter Fiction Transworld, £20