The Other Side of Silence is structured in passages headed “Real” and “True”. Written in the first person, it tells the story of 12-year-old Hero, an elective mute. Hero has grown up struggling for air in a family of prodigies, and keeping silence enables her to breathe. When I met Margaret Mahy in London last week, she told me: “By being silent, by taking things into herself and not surrendering them, that’s her way of being famous ... it’s a way of manipulating the world.”

Hero’s “real” life is lived at home, in a family whose complex dynamics are conveyed in deftly choreographed dialogue, which dances round Hero’s silence at the centre. Her “true” life is lived in her imagination. Climbing in the trees in the wild garden of the old Credence house, Hero imagines herself to be a wild creature, a Mowgli. At the book’s harrowing climax, she discovers in the attic of the house a real wild creature, a closet child - Miss Credence’s illegitimate daughter, a child who, in Margaret Mahy’s words, “has had silence imposed on her.”

At this moment, “real” and “true” fuse, and Hero finds her voice. As in several of Margaret Mahy’s novels, the absorption of the child’s fantasy life marks the beginning of adulthood. Margaret Mahy describes the book as “a traditional Gothic story,” but this seems a characteristically modest assessment of a novel that, for all its Gothic trappings, zeros in so mercilessly on real human suffering.

For instance, after Hero has discovered the child in the attic there is a very powerful, understated scene between her and the mother. Miss Credence says: “I thought of killing her when she was born . . . All I would have had to do was to hold a pillow over her face for a few minutes, and it would have changed everything. I could have planted her out under one of the trees, and no one would ever have known. But there you are. I was sentimental. I didn’t do it. Well, not very often, and never enough to kill her.”

When I asked Mahy about this passage, she said: “I think that’s an absolutely ferocious moment in the story. I remember writing it, and thinking, although it’s so casual, the implication of the years that had gone before is horrific. ”

This quality of ferocious implication is typical of Mahy’s novels. Most writers for young adults concentrate on moments of crisis in adolescence, but Mahy welcomes and celebrates those moments as necessary resolutions of inner pain. She takes her characters - Angela, in The Catalogue of the Universe or Jonny in Memory - right out to the brink. Her father was a bridge-builder, and she has inherited from him, it seems, an unerring sense for the point of breaking strain. Typically, Jonny, after his crisis, “felt as if he had died, and had then been born again with his own death dissolved into his blood. ”

What does Mahy think about this quality of recklessness and desperation, in her characters and in her work? “I suppose I think it’s a bit of a romantic idea that everyone comes to the edge of the cliff and has to plunge over in ruin. A lot of people save themselves in part, or they do save themselves eventually. That’s something that’s probably within my own experience, rather than crashing in ruin . . . I think really imaginative people take a lot of risks. I think they take a risk in the way they choose to interpret their lives and the way in which those interpreted lives clash with consensus reality. ”

In a brilliant series of novels from The Haunting in 1982 to Memory in 1987, Margaret Mahy seemed always to be pushing to the limits what could be dared and what could be achieved in the young adult novel. Later books, such as Dangerous Spaces, Underrunners, and now The Other Side of Silence, seem to have retreated in age level and ambition from the high water mark of the starkly uncompromising Memory.

Margaret Mahy, who defines herself first as “a reader” and second as “a tradesperson”, claims that this is what today’s market demands. Of The Other Side of Silence, she says: “I originally started writing this book for people of about 15 and was encouraged to bring it down a bit.”

She was born in Whakatane, a country town in New Zealand, in 1936, and says she had “a happy childhood in a lot of ways. I read a lot, my parents loved me . . . I constructed stories. I told my sisters and brothers how to behave, and what sort of stories they should play. I wrote stories from the time I was quite small.”

After university, she trained as a children’s librarian, and although she never stopped writing it was not until 1969 that her first book, A Lion In the Meadow, appeared. A decade later she became a full-time writer and is now one of today’s most prolific authors for children and young adults.

It is part of her professionalism to be responsive to the needs of editors and readers, but it would be a shame if the forces of commercial caution were to limit for long a writer in whose best work no holds are barred .

In that best work - which is always packed with humour as well as dark secrets and terrible resolutions - Mahy has extended the boundaries of fantasy writing. The Catalogue of the Universe and Memory are, in effect, fantasies without magic. They are books about how the world is shaped out of the imagination.

When I raised the question of her use of scientific ideas, she quoted Einstein: “When I consider myself and my habits of thought, I come to the conclusion that my gift for fantasy has been much more useful to me than my power of absorbing positive knowledge.” The Ionian scientists were, says Tycho, in The Catalogue of the Universe, “imagination men”: they had to be, for “the square root of two is an irrational number.”

The irrational basis of rationality is Margaret Mahy’s prime subject: the collision of the real with the true.

oThe Other Side of Silence. By Margaret Mahy. Hamish Hamilton Pounds 9. 99. 0241 135516