The Voyage of the Dawn Treader strikes me as the most melancholy of the Narnia books I've read so far; it benefits from being the one book in the series without a clear villain and is simply a picaresque voyage to the end of the world (aka Aslan's country).
Like Edmund in The Lion, the Witch and the Wardrobe, newcomer to Narnia Eustace Scrubb undergoes a change of character as the narrative proceeds. Most of the book takes place aboard the ship the Dawn Treader, led by Prince Caspian, who's attempting to find the seven lost Lords of Narnia (see the book, Prince Caspian). The islands explored and adventures undertaken are by turns disturbing, exciting and amusing. A standout is Lucy Pevensie's creepy journey through the upstairs quarters of a magician named Coriakin.
The novel culminates in a remarkably metaphysical setting, as the Dawn Treader can literally go no further into the end of the world and Lucy, Edmund, Eustace, and Reepicheep journey on into the strange, dreamlike, lilly-filled water in a small boat. One could argue that Aslan's country is as much a metaphor for mortality as heaven and Reepicheep's mysterious fate brings a poignancy to bear that most children's novels don't attempt.