The Runaway Troll by Matt Haig
From the bestselling author of A Boy Called Christmas, The Girl Who Saved Christmas, Father Christmas and Me and The Truth Pixie.
Samuel's not allowed to enter Shadow Forest. But what's to stop the forest from coming to him?
There's one thing every troll child dreads more than anything - and that's the Betterer, the evil troll in Shadow Forest who loves to punish his fellow trolls for their grubby and stupid habits in lots of horrible ways.
When one troll boy escapes the forest to be with his favourite human, Samuel Blink, the Betterer isn't happy. Until he realizes that the most useful thing to have in his Bettering Tower would be a real life human child. So he sets out on a quest into the human world to kidnap Samuel and the troll friend he's been hiding in his wardrobe . . .