Abigail's Mirror

When you look into a mirror, you should always know who is going to be looking back. But what if it isn't you? In Abigail's Mirror, Ravine discovers a curious mirror in an old shop and, for reasons she doesn't understand, she knows she has to buy it. Then, when she is carrying it home, she discovers someone else looking back at her from the mirror.
Fresh off a ghostly adventure with her friend Derek, she tries to get him involved to investigate what's going on with this mirror. Like it or not, the mirror - and the supernatural mystery behind it - is going to consume both of them.

Although the mirror isn't connected to the mystery they solved a few months earlier, they are able to use some of what they learned. They find themselves in danger and in the midst of another mystery, one that teaches them about time travel, and whether it's possible to alter the past.