The Thief of Mirrors (Enchanted Emporium #4)

A special meeting of the Enchanted Emporium's shopkeeper families is actually a deadly trap set by the evil Semueld Askell, but before he can come to the rescue Finley McPhee needs to figure out where the meeting is being held--a task made more difficult because his older brother refuses to return the magic key that rightly belongs to Finley as the defender of the Emporium.