Frances Hardinge speaks of shock at winning Costa book prize with 'The Lie Tree'

Frances Hardinge has described how she was "stunned" to become the first children's author in 14 years to win the prestigious Costa Book of the year award. Her seventh novel, The Lie Tree, beat Andrew Michael's Gothic horror book titled, The Loney, to win the £30,000 prize for 2015.

Philip Pullman was the last children's writer to win the Costa prize, which is restricted to writers in the UK and Ireland. He won it in 2002 for The Amber Spyglass. The Lie Tree is a 19th century detective novel that portrays a teenage girl, Faith Sunderly, who sets out to uncover the truth of her father's mysterious death.

"I'm still stunned, but now with added sleep deprivation," Hardinge told the Guardian. "... had a feeling of unreality as if I'd seen a cat walk past on its hind legs".

After accepting the awards, Hardinge thanked people who were involved in publishing the book and said, "I probably should have thanked more people – but I didn't want to get all Oscar on them."

Hardinge, an Oxford University graduate and previously worked as a technical author for a software company, has written six other books.

James Heneage, chair of the judges' panel, said: "Part horror, part detective, part historical, this is a fantastic story with great central characters and narrative tensions." Heneage pointed out that Faith's search for answers was set in the context of a male-dominated society in which women were "seen and not heard".

Hardinge has "brilliantly articulated" the voice of a precociously intelligent 14-year-old in a male-dominated Victorian society," he said.

"There is something deep inside me that wants it to be a bestseller. It's a book to read in one or two sittings. It is a real page turner," he added.

2015 Costa award winners

First novel award: Andrew Michael Hurley - The Loney (John Murray)

Novel award: Kate Atkinson - A God in Ruins (Doubleday)

Biography award: Andrea Wulf - The Invention of Nature (John Murray)

Poetry award: Don Paterson - 40 Sonnets (Faber)

Children's book award: Frances Hardinge - The Lie Tree (Macmillan Children's Books).