Reluctant heroes are a bit overdone, but this one is honest and plausible (though, as all reluctant heroes, a bit tedious). That’s the good part, possibly. The characters are not exactly one-dimensional, but the book is certainly reluctant to show you more than one dimension at a time.
Protagonist from modest upbringing on a deliberately peaceful fantasy island sets out to solve riddles that this world is structured around. (The riddles are a very strong choice both for style and for world-building. I enjoyed them throughout the trilogy). He finds out that the world is not run the way he thinks it is, and it’s all a mix of extremely slow and dramatic. Good twist, good drama, but everything is so very slow and anti-climactic no matter what happens.
I’m not sure how much of this is the book’s age. Or rather, I think the book’s age gives way to the oddly distanced slow pace, but I don’t think its age is all the reason there is. It reminds me of Earthsea, though le Guin is never quite this distanced.
Bonus points for learning to become a tree by thinking like a tree.