Prince Caspian

Prince Caspian is a novel for children by C. S. Lewis, first published in 1951. It was the second of the Chronicles of Narnia to be written, though in the overall chronological sequence it comes fourth.

Warning: wikipedia contains spoilers

The four Pevensey children, Peter, Susan, Edmund and Lucy, return to Narnia and find that many years have passed since they reigned there as kings and queens (in The Lion, the Witch, and the Wardrobe). The old magic, including talking animals and trees, has been lost. The children are drawn into the struggle to restore the rule of the young Prince Caspian, rightful heir to the throne of Narnia.

Caspian reappears in two of the later books in the series: The Voyage of the Dawn Treader and The Silver Chair.