The Castle of Llyr by Lloyd Alexander - USED

In the imaginary kingdom of Prydain, Princess Eilonwy must leave her friends to go to the Isle of Mona for training as a proper princess. Shortly after her arrival, something sinister and secret befalls her. Eilonwy's loyal friends--Taran, the Assistant Pig-Keeper; Fflewddur, the bard; and Prince Rhun, her intended husband--realize her peril and set out on an exciting mission to rescue her. ALA Notable Children's Book.