Inspired by the Cornish legend of the Mermaid of Zennor, its narrator Sapphy, is a young teenager whose father is lost at sea. Everyone including their mother believes him dead, but in fact he, like his children, shares the blood of the Mer, and can live underwater. When Sapphy and her brother Conor meet the beautiful, mysterious Faro in a hidden cove, it is the beginning of a friendship at once exhilarating, dangerous and magical.

“Cornwall is a very strong, seductive, secret place,” says Dunmore. “There’s that interface between granite and the sea…”

She was sitting on a beach very like Sapphy’s when she had the idea for Ingo. “I thought, what if you could just slip through the skin of the water, what would be there? I think Ingo is drawn from that profound feeling that the sea is both dangerous and beautiful, that it both gives and takes.”

Her heroine’s adventures in Ingo – where she can swim without breathing, and talk to dolphins, whales and sharks as well as the Mer people in a language derived from Cornish - is wonderfully described by an author who morphed from distinguished poet to an adult novelist who won the inaugural Orange prize for A Spell of Winter and has been short-listed for the Whitbread. Distinguished by their descriptions of heart-break, jealousy, hunger and passion, there is a fairytale quality to Dunmore’s adult work, which has reached its full flowering in her novels for children.

Sapphy is torn between the ordinary world of Air, guarded by the mysterious, earthy wise woman Granny Carne, and the intoxicating underwater world of Ingo. The combination of the natural and the magical is a classic recipe in children’s fiction, but each is described with a poet’s precision, so that the reader, too, is torn between land and sea. The Mer we encounter are the opposite of the pretty, silly mermaids of fairy-tale; and Faro, powerful, beautiful, brave and thoughtful has to be one of the most seductive figures in new fiction.

“I didn’t want a sense of them being deficient, of having half a body,” Dunmore says. “I repudiate the idea of a fishy tail – what they have is strong, seal-like. They are captivating and strange. Yet Faro is also moving out of his world to befriend Sapphy. Both of them are moving out of childhood and meeting people on their own terms, not through mediators.”

A keen swimmer herself, she has a foot in many worlds,  being familiar to London literary parties partly as a result of Chairing the Society of Authors last year, but also guarding her life in the West Country. Her productivity (9 adult novels, 17 children’s novels, 9 poetry books and numerous radio plays, critical essays and reviews) is startling, not least for a consistently high quality that has made her a favourite of reading groups. She has a grown-up son and step-son and a 13-year-old daughter; married to a lawyer in Bristol, she appears to have an enviable work-life balance, writing in a studio flat high above the city during school terms, then transferring to her beloved Cornwall for holidays.

The Deep is an especially exhilarating read, both for its emotional intelligence (the children meet their lost father’s new baby by his Mer wife) and its sense of danger and suspense. A complex web of friendships and loyalties troubles Sapphy and Conor even before an ancient monster, the Kraken, wakens and demands the traditional sacrifice of a Mer child. Only Sapphy, Faro and Conor, helped by a maternal whale, are brave enough to go down into the crushing darkness to fight the Kraken, and even they may not be able to outwit another kind of evil, from an ambitious Merman, on their return.

The second of four children, Dunmore was born in 1952 and grew up in a number of different places in Britain. This gave her an interest in exploring what it might be like for a family like Sapphy’s, rooted in one place for generations. Yet the mermaid part of her trilogy came from remembering how, as a child, “there seemed to be many more possibilities. I thought I might become a horse, or a dolphin, or a boy. It sounds crazy, but children don’t realise their identity is going to be fixed, and pinned down.”

Dunmore is intensely sympathetic, both as a writer and as a person; her unpretentious warmth is matched by dedication to the art and craft of writing, and a deep feeling for the sea.

“The sea isn’t a commodity, it isn’t a territory, as many think. Everything we do to the environment has a consequence there, which you don’t see immediately but which builds up. It’s the last frontier, which has immense power.”

She thinks the books are inspired by an early memory of “complete fear”, when her own father swam out, and temporarily disappeared from view round a Cornish bay. Today, she lives as much as possible within sight and sound of the Atlantic, describing the enchantment of finding a secret colony of seals, humping in and out of the water.

“I’m interested in the pivotal moments in people’s lives, when the weight shifts and they go one way or another,” she says. Her own oscillation between the worlds of adult and children’s fiction are just as rewarding and remarkable as her heroine’s double identity in the worlds of Air and Ingo. Neither should be forced to make a choice between the two.

The Deep by Helen Dunmore is published by HarperCollins.

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