Wintersmith, by Terry Pratchett

Wintersmith isn't a "Discworld novel", but "a story of Discworld": a Terry Pratchett story that diverges from the mainstream of his fantasy fiction, but brings with it some Discworld characters. It is also the third in a series of novels featuring a trainee witch, Tiffany Aching, first seen in The Wee Free Men.

Ostensibly, they're aimed at the teenage market, but I prefer them - their sense of space, the reduction in the number of characters. They resemble Ted Hughes's The Iron Man but are far less tendentious, and sparkle with ideas. Pratchett's cerebral cortex is hyperactive enough to power the national grid.

There are two great strands of imagination in the novel, his 35th. The first deals with superstition, and what Pratchett calls Boffo. He has witches who wear pointy hats and fly on broomsticks. But his witches know that, although their powers are real, they won't be credited unless they play along. Their powers depend on bluff. If they don't behave like traditional witches, and if they don't let rumours do the rounds about their disturbing secrets, they won't be accepted as witches at all.

Miss Treason, to whom Tiffany is the sorceress's apprentice, has plastic webs to keep punters agog. After all, as she says, "I didn't get where I am today by wearing a woolly bobble hat and a gingham apron. I look the part." Even witches indulge in spin.

The second imaginative leap in Wintersmith has the elemental creator of storms and blizzards behaving in a comically adolescent way. The Wintersmith may be able to freeze rivers, but when he falls for headstrong Tiffany at a forbidden dance, he acts like a schoolboy with a desperate crush. He makes all snowflakes look like Tiffany; he makes icebergs in her image. What ensues is wonderful folklore, which parodies itself even as it develops. Pratchett is a great myth-raider, but he always grounds his myths in whimsy.

Light entertainment is provided by an animated cheese called Horace, and the Nac Mac Feegles, six-inch Scotsmen with a good line in vaudeville patter, prepared to give even Charon grief - they haggle over return tickets on the Styx. Pratchett is great, not because he doesn't know where to stop, but because he knows where not to stop. He packs teenage sexuality, elemental mystery, and outright panto into the same space. Wintersmith is in every sense fantastic.

Bill Greenwell's 'Impossible Objects' is published by Cinnamon