It is only the second time in the award's history that a children's book has taken the prize, the first being Philip Pullman's The Amber Spyglass in 2001.
It was also announced that Daniel Murphy took home the Costa Short Story Award, and £3,500, for his work Rogey. The two runners up, Erin Soros and Annalisa Crawford, took home £1000 and £500 respectively.
The other categories for the Costa Book Awards were also announced earlier this month, and it was a second-time win for two of the five finalists.
The category winners are:
The Loney by Andrew Michael Hurley - Costa First Novel Award
A God In Ruins by Kate Atkinson - Costa Novel Award
The Invention of Nature by Andrea Wulf - Costa Biography Award
40 Sonnets by Don Paterson - Costa Poetry Award
The Lie Tree by Frances Hardinge - Costa Children's Book Award
Click through for more on this year's prize winners...