It's Not Her by Mary Kubica

I love this author for her twisty shocking thrillers and especially the chilling fact that this could have happened in real life! Told in alternating chapters from the missing girl, Reese, in the days before, and Courtney, Reese's aunt, after the unaliving when she's desperately trying to find Reese while suspicious of all the side characters in the story. As with every mystery, I knew that the most obvious suspect would be a red herring, but there were sooo many suspicious characters, all with secrets of their own! I never knew what was going to happen until the final twist.