A Wizard of Earthsea
by Ursula K. Le Guin
(5) Tree Line
Ursula K. Le Guin actually wrote A Wizard of Earthsea for readers eleven years old and up, but that doesn't mean the book's super easy (source). She actually has a few essays in other books about how literature for young readers is OK when it's challenging – that you certainly shouldn't assume "young" = "dumb." So Le Guin is not going to condescend to you and treat you like you're not smart. But that means you might have to do some extra work to keep up with her, eleven years old or forty.
Actually, we all do have to do that work because this book is somewhat uniquely written. There's the whole fairy tale/oral folklore issue, but there are other issues, like some long sentences, some words that seem to have a different meaning than you're used to, and so on. But don't worry – this is all still meant for eleven-year-olds, and there are plenty of easy sentences sandwiching those hard ones. Just keep the dictionary at hand.