The Magician's Boy — Susan Cooper

Only a child can find the way to bring Saint George back to the play.

The Boy works for the Magician, and he wants more than anything to learn magic. But the Magician always says, "Not yet, Boy. Not till the time is right." So the Boy has to be content with polishing the Magician's wand, taking care of the rabbits the Magician pulls out of hats, and doing his favorite job: operating the puppets for the play Saint George and the Dragon, which the Magician always performs as part of his act.

Until one day the Saint George puppet disappears, and the angry Magician hurls the Boy into the strange Land of Story to find Saint George. His quest is full of adventures with oddly familiar people, from the Old Woman Who Lived in a Shoe to the Giant at the top of Jack's beanstalk. But the Boy's last adventure is the most amazing of all -- and changes his life forever.