The Silver Chair by C.S. Lewis

Back Cover: NARNIA … where owls are wise, where some of the giants like to snack on humans (and, if carefully cooked, on Marsh-wiggles, too), 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 Rilian is to be saved.

Enter this enchanted world countless times in The Chronicles of Narnia.  There are seven books in all.