Wizard of Earthsea is about a boy, really, named Ged. He is taken as an old wizard's apprentice, but his talents exceed what the old wizard can teach him. Ged goes to an island known for teaching wizards, and he invokes something from the dark side of the world. This book is about Ged trying to get rid of this creature he summoned, and suceeding, but did not come away from it without a lesson learned. This book was my favorite out of all the others Ursula has written.
Ursula K. LeGuin was known as one of the most influential fantasy writers of the modern age. Here are some other authors that are considered the same:Robert Jordan,Terry Goodkind, Orson Scott Card, Terry Brooks, Tad Williams, & Terry Pratchett.
If you like a book about wizards and magic that isn't childish, then you might try this book.