The Silver Chair by C.S. Lewis

Jill and Eustace must rescue the Prince from the evil Witch. NARNIA...where owls are wise, where some of the giants like to snack on humans, where a prince is put under an evil spell...and where the adventure begins. Eustace and Jill escape from the bullies at school through a strange door in the wall, which, for once, is unlocked. It leads to the open moor...or does it? Once again Aslan has a task for the children, and Narnia needs them. Through dangers untold and caverns deep and dark, they pursue the quest that brings them face to face with the evil Witch. She must be defeated if Prince Rillian is to be saved.