Above the Veil - Garth Nix

The Seventh Tower Bk. 4 Fourth of the thrilling fantasy adventure series set on the Dark World, where society is ranked according to its colour clan and the most precious commodity is light. Tal and Milla are no longer alone in their quest. They have been joined by the rebel Underfolk led by Crow, Tal`s old foe. But now they must all work together to discover the secrets of the Castle and the Dark World they all inhabit before it is too late. The darkness is growing deeper. The shadows are growing stronger. And Tal and Milla are in greater danger than ever before!