The Book of Three (The Chronicles of Prydain) by Lloyd Alexander
This is the first book in the Prydain Chronicles. It is an excellent book for younger readers that like fantasy, but it is also great for older readers. It is very appropriate and not only delivers a great story, but also does a good job of teaching the importance of growing up and learning responsibility.
Although Taran is an orphan who spends his days caring for the prophetic pig Hen Wen under the watch of the Enchanter Dallben, he desires nothing more than to do great deeds and earn glory. He soon learns that glory isn't exactly what he thought. When Hen Wen goes missing, it is up to Taran and his unlikely allies to find her before she falls into the hands of the Horned King and his master Arawn Death-Lord.