If YOU FIND CHILDREN scrutinising lonely beaches this summer, they may be looking for signs of Ingo. Cornwall has long been the stuff of stories, from Rosemary Manning’s tale of a girl’s friendship with a dragon in Green Smoke, to Daphne du Maurier’s romances, but Helen Dunmore’s Ingo series, about a sister and brother’s discovery of the Mer people, could become just as well known.

Inspired by a Cornish mermaid legend, the story is narrated by Sapphy, a teenager whose father is lost at sea. Everyone believes him dead but, like his children, he shares the blood of the Mer, and can live underwater.

When Sapphy and her brother Conor meet the mysterious Faro in a hidden cove, it is the beginning of a dangerous and magical friendship. “Cornwall is a strong, seductive, secret place,” Dunmore says. “There is that interface between granite and the sea . . .” She was sitting on a beach much like Sapphy’s when she had the idea. “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 – in which she can swim without breathing, and talk to dolphins, whales and sharks, as well as the Mer people – is wonderfully described by a distinguished poet and adult novelist who won the Orange Prize for A Spell of Winter and has been shortlisted for the Whitbread. With their descriptions of heartbreak, jealousy, hunger and passion, there is a fairytale quality to Dunmore’s adult work that has flowered in her novels for children.

Sapphy is torn between the ordinary world of Air, guarded by the mysterious, wise Granny Carne, and the underwater world of Ingo. The combination of the natural and the magical is described with a poet’s precision, so that the reader is torn between land and sea. The Mer are the opposite of the pretty, silly mermaids of fairytale; and Faro, powerful, beautiful and brave, is 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. Faro is also moving out of his world to befriend Sapphy. Both are moving out of childhood and meeting people on their own terms, not through mediators.”

Dunmore, too, has a foot in many worlds, being familiar to the London literary scene as a past chair of the Society of Authors, but also guarding her life in the West Country. Her productivity (nine adult novels, 17 children’s novels, nine poetry books) is startling. With a grown-up son and stepson and a 13-year-old daughter, and married to a Bristol lawyer, she has an enviable work-life balance.

The latest in the series, The Deep, is especially exhilarating for its emotional intelligence (the children meet their father’s new baby by his Mer wife) and its danger and suspense. When an ancient monster, the Kraken, awakes to demand the sacrifice of a Mer child, only Sapphy, Faro and Conor are brave enough to go into the darkness to fight it.

The second of four children, Dunmore was born in 1952 and grew up in a number of different places. This gave her an interest in what it might be like for a family like Sapphy’s, rooted in one place for generations. Yet she also remembers 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’s unpretentious warmth is matched by dedication to the 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 fear when her father swam out and disappeared from view around a Cornish bay. Today, she lives as much as possible within sight and sound of the Atlantic. “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 adult and children’s fiction are as rewarding and remarkable as her heroine’s double identity. Neither should have to make a choice between the two.

THE DEEP by Helen Dunmore

HarperCollins, £12.99; 336pp

Buy the book here for the offer price of £11.69 (free p&p) timesonline.co.uk/booksfirst

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