Books of the Change trilogy

Sal and his friend Shilly have been joined by a third refugee, Skendar, by the time they make their way to the remote island that is the home of the famous, feared Haunted City, home of the Sky Wardens. Sal's great aunt, the Syndic, is also there, and she is determined to keep Sal under control, even if she has to imprison him to do it.   But there are greater threats even than that. The ghosts that live behind the ancient glass of the City are restless....