The Seventh Tower Series Novels

The Violet Keystone (The Seventh Tower 6) by Garth Nix
Into Battle (The Seventh Tower 5) by Garth Nix
Aenir (The Seventh Tower 3) by Garth Nix
Castle (The Seventh Tower 2) by Garth Nix
The Fall (The Seventh Tower 1) by Garth Nix