Ghostfire — Christopher Golden; Thomas E. Sniegoski

IS LEANDER THE ENEMY?
After shunning Timothy since his arrival in Arcanum, Parliament has finally turned to him for help. Raptus, the vicious leader of the Wurm, is determined to break through the magical barrier that separates his world from Sunderland and exact vengeance upon the mages. Parliament needs Timothy's kind of magic -- the magic of invention -- to prep are for the inevitable battle with the Wurm.
Timothy sets out with Leander and an exploratory team to the site of the barrier, but over the course of the trip Leander's behavior goes from erratic to dangerous -- so much so that Cassandra is named Grandmaster of the Order of Alhazred. Can Timothy and Cassandra find the source of Leander's transformation before it destroys them all?
The saga continues in Wurm War.