The Nightmare Before Christmas: Mirror Moon #4
When the master of scares Jack Skellington has a dream about bringing his expertise in spookiness to the rest of the holiday worlds, he appoints Sally the official Halloween planner so he can spend his time on his newest invention. But as Sally coordinates with the Halloween Town residents, it becomes clear that something is very wrong. It's almost as if there's a secret doppelganger sneaking around behind her back! Can she get to the bottom of this mystery in time to save the celebrations?