The Nonexistent Knight

Agilulf is the ideal knight, characterized by his deep loyalty, reverence, and chivalry. He serves with Charlemagne's army, roaming through the world, setting things to rights. And yet the truth is that Aguilulf does not exist; Aguilulf is merely a suit of armor with nothing inside.

The Nonexistent Knight is the most grownup of the 1001 Children's Books You Must Read that I have read so far. There are thoughtful ideas of what it means to be a person of virtue, as well as clever satire, in this story.