Her first novel was an international bestseller, now Emma Healey has a new book out which transforms a mother's tense relationship with her teenage daughter into a must-read mystery.

Lana is missing for four days. When she returns her family is, at first, delighted. But where has she been? Why is she bruised and bleeding and afraid of the dark?

Norwich novelist Emma Healey is not afraid of tackling difficult subjects. Her first book, Elizabeth is Missing, was told from the point of view of a personality ravaged and fragmented by dementia. It has sold more than a million copies around the world, won the Costa first novel award and is being made into a BBC series. Her second novel, Whistle in the Dark, is narrated by Jen, the mother of a suicidal 15-year-old.

Despite the darkness of parts of the story, this is a book full of wit and warmth.

The text is cut into short chapters, compelling the reader deeper and deeper into the mystery at the heart of the plot, along trails of clues and characters and possibilities.

'I wanted to add to Jen's paranoia. I wanted to keep the reader guessing,' said Emma.

Was Lana kidnapped, brainwashed, lost, deliberately hiding?

Layers of fairytales and legends lurk just beneath the everyday realism, intruding into a 21st-century family life of laptops, shopping trips, careers, school, housework and meals, and ramping up the desperation of a mother trying to discover the cause of her daughter's depression, disappearance and rediscovery.

As Jen searches for answers she begins seeing every event as symbolic, every character as sinister. A cat which may or may not be there, or alive, slinks in and out of the family home; the boundaries between science and myth, contemporary life and ancient wisdom become blurred. A simple scene of a teenager picking at pomegranate seeds in a London kitchen resonates with ancient fables of seasons, harvests and an underworld.

At the centre of it all is a tense yet tender, mother-daughter relationship.

Speaking from her home in Norwich, with her beautiful baby daughter playing at her feet, Emma explained that the story of Whistle in the Dark began when a woman went missing for 17 days in Australia. Emma was fascinated by the speculation which followed her reappearance.

'I'm really obsessed with motive,' said Emma. 'I had written the first third of one book, then the first third of another, and was getting to the point where I would have to pay an advance back.'

Then she found herself transforming the mystery of a real-life disappearance into a novel exploring family life, depression and beliefs.

'I went through a period of teenage depression and I thought maybe I could start to talk about that. I had been avoiding it,' said Emma.

'At 15 I was in a friendship of three and that really fell apart. And there was the pressure of exams. I was exhausted. I didn't want to engage with the rest of the world because I was so overwhelmed, so tired.'

Suicidal, she was about to be admitted to a mental health unit when a doctor questioned the decision and suggested complete rest. She dropped out of school and did little more than read romantic fiction for a year until she had recovered enough to begin a degree in bookbinding, and eventually an MA in creative writing at the University of East Anglia.

Although depression is a central theme, the book sparkles with humour and pitch perfect exchanges, brimming with affection and irritation, between anxious mother and angsty daughter.

Part of the story is set on a painting holiday in the Peak District and Emma's research included a day caving in the Peak District. 'It's just exhausting, and scary. You feel terrified the whole time,' said Emma. 'There is no light so you can't tell how long anything is lasting and you are wet all the time. There are no sounds of life. Afterwards we were just marvelling at the sunlight and the green fields, the sheep, the rabbits...'

Whistle in the Dark is dedicated to Emma's mother and Emma became a mum herself while finishing the novel. Baby Cora was born last summer. After a difficult birth, with Cora spending her first few days in special care, they brought her back to their new home just in time for a summer street party.

It was a busy year. Emma, now 33, and Andy married at St John, Timberhill, Norwich, holding their wedding reception where they first met as colleagues in Waterstones bookshop, in the Royal Arcade - now a Jamie's restaurant.

Emma and Andy adore living in Norwich. 'It's so easy to live here,' said Emma. 'It's just lovely. I feel like it seems to breed happy people. There's a level of contentment that you don't get in London. We have everything.'

Whistle in the Dark, by Emma Healey, is published in hardback by Penguin Random House, for £12.99.