This book makes a big deal out of mirrors, which is kind of a shout-out to Through the Looking Glass. In this story, Alice actually goes through a mirror and winds up in a totally new world. Sound familiar?
Coraline goes through a door to get to her other world, but her parents and three ghost children are trapped in a mirror. And when you think about it, what happens when you look in a mirror? Everything looks the same... but different. Right? This is exactly how Coraline feels when she walks into the other world – like it's familiar, but something's just not right.
Even the other mother knows that mirrors don't represent reality:
The other mother smiled. "Mirrors," she said, "are never to be trusted." (6.78)
Not that we trust the other mother, but it turns out she's right.