LONDON (AP) — "The Lie Tree," a Victorian mystery for young readers, has won Britain's Costa Book of the Year prize — a rare triumph for youth fiction at a major literary prize.

Author Frances Hardinge said she was surprised and delighted to receive the 30,000 pound ($50,000) prize Tuesday night. She said that "sometimes children's fiction is seen as a bit lightweight, in a way that I think is not deserved."

Open to writers form Britain and Ireland, the prize selects a champion from winners in five categories: novel, first novel, biography, poetry and children's book.

Hardinge beat Kate Atkinson's wartime novel "A God in Ruins"; Andrew Michael Hurley's debut horror novel "The Loney"; Andrea Wulf's biography of scientist Alexander Von Humboldt "The Invention of Nature"; and poet Don Paterson's "40 Sonnets."