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.

Arthur de Caldicot, on his way to becoming a man, witnesses the horrors of the Fourth Crusade in Venice and Zara, as well as the downfall of King Arthur's court, in his seeing stone.