What is the moral of The Book of Three?
One moral of the story is that ordinary people are capable of great deeds. Taren, the protagonist, is the most ordinary person in the story; he has no special skills or magical powers, but he is driven to become a hero because of circumstance, and he finds himself able to overcome great obstacles because of his drive and ambition. Despite his desire for adventure, he slowly loses his taste for it as he witnesses violence and cruelty. His desires are also looked down on by the other members of his party, as in this scene where Princess Eilonwy explains that the sword Dyrnwyn can only be drawn by a person with royal blood:
"...that's the name of the sword; here it is again:
DRAW DYRNWYN, ONLY THOU OF ROYAL BLOOD, TO RULE, TO STRIKE THE...
"Something or other," Eilonwy went on.
"I wouldn't dare to draw it, and I don't intend letting you, either. Besides, it says only royal blood and doesn't mention a word about Assistant Pig-Keepers."
(Alexander, The Book of Three, Google Books)
Despite this, Taren is able to draw the sword part-way through sheer persistence and, probably, because of his fundamentally good heart and soul. This action distracts the Horned King long enough for another character to kill him and save Taren and Eilonwy. Although of common birth, and without skills or magic, Taren was capable of valorous acts, and was in the end partly responsible for saving the land from the Horned King.