Often compared to Tolkien, Ursula K. LeGuin is a true visionary. This book tells the deceptively simple tale of a kid named Ged who lives in a magical realm called Earthsea. He is destined to be the greatest wizard in the world, but still has a lot of growing up to do. After meddling with the powers of chaos, he accidentally summons an eldritch creature from the outer dark, and has to put it back there. In the process, LeGuin tells a compelling coming of age story, and by combining this with a high fantasy story, she sets the groundwork for Harry Potter (Hogwarts is suspiciously similar to the Roke Island wizard academy Ged trains at), Eragon, The Legend of Zelda, (perhaps unconsciously) Star Wars, and many, many others.
The thing that defines this from other high fantasy works like The Lord of the Rings is how well developed the character of Ged is. Whereas Frodo (and even Harry Potter sometimes) got through their adventures simply by "being there", Ged has to constantly be taking responsibility for his actions because the otherworldly menace that haunts him exists partly because of his own shortcomings, making this also one of the first "existential" high fantasy books. Ged's own angsty brooding over his own failings did, and will continue to resonate with modern day teenagers. On top of all that, the cast of characters has a surprising amount of ethnic diversity, for a fantasy novel. Whereas A Song of Ice and Fire or something along those lines usually has a nearly all white dramatis personae, Earthsea has a myriad of different cultures and ethnic groups represented. Although it may sound like I'm bashing fantasy, I'm not. I love it, which is why I so enjoyed A Wizard of Earthsea. It revels in and codifies some of the cliches of its own genre. Not only a good fantasy tale, but also a cracking good YA novel.