Book 5 in the Chronicles of Narnia series by C.S. Lewis.
While visiting their nasty cousin Eustace, Lucy and Edmund are transported back to Narnia, to the sailing ship of Caspian, now king of Narnia. To their dismay, Eustace gets dragged along with them!
Caspian has set sail for the Eastern Islands to find seven lords who were banished there by his evil uncle. But the Eastern Islands are full of strange places and people, and Eustace learns some particularly difficult lessons.
Dragons, treasure, and a quest will bring them to Aslan himself!