The Violet Keystone by Garth Nix
Tal has found the violet keystone-half of it, anyway-and freed a guardian of one of the keystones. Since he has the Violet Keystone, he is technically the new emperor. With Milla, he must find the man behind the destruction of the veil so the veil can be made whole again.
When they chase the culprit into Aenir, they face many challenges. Will they fix the veil and reunite Tal's family in time?