Just to add a couple of thoughts to this.

Earthsea is often described as being rather mature despite being YA. I agree. There's a clarity and immediacy to Le Guin's writing style that cuts straight to the point while also providing clear images and characters which is combined with deep and arguably universal themes and sometimes nice allegory.

Additional to this, the series goes or progresses in surprising directions. As is so often the case, Le Guin didn't intend to write as many books as she did, but used the opportunity to do interesting or personal things with each book. While there's a continuity throughout the whole series, it's not a simple or single story but rather multiple stories with large or important intersections. You could for instance stop any time you like and not really miss out on any satisfying climactic ending.