I love the Seventh Tower. It's by Garth Nix, who's got some other stuff that I really like. It's the worldbuilding that reels me in. It's definitely written to a younger audience, think the younger end of middle grade, but I still like it. The plot and characters are pretty standard, but become interesting when it's affected by the worldbuilding.
The worldbuilding is based around different applications of light and shadow. Interplay between light magic cast from sunstones and living shadows. It's nothing terribly deep, leaving a lot of how it's applied to the imagination, but that's the way I like it for the most part. The setting is the Dark World, where a massive Veil lays over the sky, blocking out the sun, leaving the world in ice and snow. But there's the Castle, an enormous, seven towered building where the Chosen live, users of light magic who live not really knowing or caring what is outside of their self sufficient society. Then the main character, a Chosen boy named Tal, after running into a bunch of problems, tries to scale one of the towers and falls off. He thus discovers not only the people that live out on the Ice, but facts that shake his belief in some of the key facts that his worldview is based on.