The Silver Chair (Chronicles of Narnia, Bk. 6)

Supposing I told you I'd been in a place where animals can talk and where there are -- er -- enchantments and dragons -- and -- well, all the sorts of things you have in fairy-tales. Scrubb felt terribly awkward as he said this and got red in the face. 'How did you get there?' said Jill. She also felt curiously shy. 'The only way you can -- by Magic.' Through an ordinary doorway Eustace Scrubb and Jill Pole, stumble upon Narnia. But it is not their own wishes that bring them to the enchanted land; it is the will of the mighty Aslan. The noble Lion sends the children on a mission to find the lost Prince Rilian, beloved son of Caspian, who was lured away by a mysterious woman ten years ago and has been missing ever since. Together with Puddleglum, the gloomy but valiant Marsh-wiggle, Eustace and Jill must brave menacing giants, and other grave dangers before delving into Underland to save the Prince. And although they have reached the end of their quest, their troubles have only begun.