The Silver Chair: The Chronicles of Narnia

Narnia is in peril, and only Eustace and Jill can help. Along with Puddlegum, a gloomy but valiant Marsh-wiggle, they are sent by the mighty lion Aslan to find Prince Rilian, heir to the throne. Their quest leads them past hungry people-eating Giants, and deep into the dark underground. But the true test for this noble band of friends comes when they face an evil witch and her deadly enchantments.

This was the fourth book written in The Chronicles of Narnia. It now stands as the sixth book in the series.