Eerie in the Mirror is a novella written by Robert James. It was the 16th book in the Eerie, Indiana book series.
Description[]
When Stanley accidentally throws a rock through a mirror in Eerie, things get even weirder than usual. Not only does everything seem just the opposite of how it was before, but also when Mitchell gets back to his house later, Stanley's already there, and he thinks it's his house! Somehow, that mirror accident created two sets of boys in a crazy, mixed up Eerie. And if Mitchell and Stanley even touch the anti-Mitchell and anti-Stanley, they'll cancel each other out. Unless they can fix things again, there'll be nothing left of the boys but a black hole!