Above the Veil by Garth Nix
Tal knows his father is the guardian of the Orange Keystone, but was trapped inside it when it was unsealed. If it is sealed again, his father will be free. If it is not sealed, he will remained trapped forever and the Veil that keeps shadows out of Tal's world will be in danger. Tal, joined by Crow, one of the Underfolk, decide it would be best to go another keystone to avoid traps. They find Lokar, the guardian of the keystone, also trapped. In order to free her so she can help, they must use the empress's Violet Keystone. But the empress is in Aenir and they are running out of time.
Meanwhile, Mila is returning to her home to report what she had seen to her people. Then she plans to give herself to the ice because she had violated the laws of the Icecarls and lost her normal shadow. Will Tal find the empress in time to save his family? What will the Icecarls do when they here Milla's news.