Arthur trilogy (Kevin Crossley-Holland)

In late twelfth-century England, a thirteen-year-old boy named Arthur recounts how Merlin gives him a magical seeing stone which shows him images of the legendary King Arthur, the events of whose life seem to have many parallels to his own.

When Arthur takes part in the Fourth Crusade and watches it fall into disarray, he sees the horror of war and looks into the seeing stone for guidance.