The Book of Three by Lloyd Alexander, 190 pages

Taran is bored with his Assistant Pig-Keeper duties, even though his charge is none other than Hen Wen, Prydain's only oracular pig. He'd rather be doing something more heroic, like making swords and learning to use them.
When Hen Wen escapes and Taran goes after her, he finds himself farther from home than he's ever been. Soon he begins to realize that heroism is no easy task. With the dreaded Horned King on the loose and King Arawn gathering the forces of evil, Taran must look past his own dreams to warn the population of Prydain-before it's too late.