A Confusion of Princes, by Garth Nix

Garth Nix is a great author. If you like Ender's Game and/or Star Wars, or just general science fiction, this is a good one!  Also check out Mr. Nix's other books: Sabriel, Lirael, and Abhorsen (the aforementioned three constitute a trilogy), his Seventh Tower, the Keys to the Kingdom series, and Shade's Children.