The Silver Chair by C. S. Lewis
Through dangers untold and caverns deep and dark, a noble band of friends is sent to rescue a prince held captive. But their mission to Underland brings them face-to-face with an evil more beautiful and more deadly than they ever expected.
The Silver Chair is the sixth book in C. S. Lewis's classic fantasy series. For over sixty years, it has been drawing readers of all ages into a magical land where giants wreak havoc and enchantment rules. This is a complete stand-alone read, but if you want to discover what happens in the final days of Narnia, read The Last Battle, the seventh and concluding book in The Chronicles of Narnia.