Samuel Blink and the Forbidden Forest by Matt Haig

When Samuel and Martha are sent to Norway to live with their Aunt Edna after their parents’ deaths, they soon learn her most important rule: NEVER – UNDER ANY CIRCUMSTANCES – GO INTO THE FOREST. She doesn’t offer an explanation, but Samuel suspects it might have something to do with a strange guidebook he finds in her attic, The Creatures of Shadow Forest. And when Martha wanders into the trees and is captured by some of the creatures described in the book, Samuel has no choice but to go in after her. What he finds there is an eerie world populated by trolls, truth pixies and other fantastical creatures.