The Chronicles of Pyrdain are, quite simply, the best children's books of all time. They take all the basics of Welsh legend, twist it a bit, and repackage it in a modern format as a fantastic coming of age story. They have magic and adventure, frightening villains and funny sidekicks, and a really well constructed world.
When I was a kid, these books are what really turned me into a lifetime reader. My third grade teacher saw that I was too advanced for what the class was reading, and had me read these instead. I'd already read The Hobbit, but I thought the tales of Taran, Assistant Pig Keeper were much much better. And they are.
If you have kids, read these to them - they're great out loud.
If you have any interest in Celtic / Wesh mythology, read them yourself. I can't think of a better primer.
If you just love a good hero's journey story that's well written and funny, read them.