The Silver Chair by C.S. Lewis

Another book I remember enjoying as a child, and still enjoy, is The Silver Chair by C.S. Lewis. Eustace Scrubb and Jill Pole, his friend, end up drawn into Narnia when they're trying to escape bullies at school. They end up on a trip to the north of Narnia to rescue the king's son. (King Caspian was a friend of Eustace in the previous book.)

Prince Rilian has been taken by an evil witch who has a plan to take over Narnia, and the kids, with the help of their marshwiggle friend Puddleglum, find this out on their adventures. But will they succeed in rescuing the prince, or will the evil witch, who can turn into a giant snake, stop them?