The Chronicles of Narnia: The Voyage of the Dawn Treader by C.S. Lewis

The Voyage of the Dawn Treader brings back the story of the Pevensies, for Peter and Susan are too old to travel to Narnia and leave Lucy and Edmund to deal with their cousin Eustace Scrubb. They are once again taken to Narnia on a ship called the Dawn Treader, owned by King Caspian the Tenth. Caspian is set on finding seven of his father's friends, and they sail through many adventures, a lot of which heavily impact Eustace's belief of the land. He even turns into a dragon in the process! After finding his father's friends, Caspian makes a new mission to travel to Aslan's country at the end of the world. This is where Lucy and Edmund find out they can also no longer travel to Narnia, and they must return home. My favorite line in the book is where Aslan says he goes by a different name in the real world, and that they must learn to call him by that name.