The Wig in the Window by Kristen Kittscher

Sophie and Grace sneak out in the middle of the night to discover the identity of a serial killer. However, they get side-tracked when they pass the school guidance counselor's house. She's yelling into her cellphone that she'll "rip their throats out" and slams a cleaver into a bag sending red splattering everywhere. The girls are convinced the counselor is a killer, but the police investigate and tell the girls they are wrong. The counselor becomes more involved in Sophie's life, and the girls are still convinced that she's done something wrong. Then, a woman claiming to be an FBI agent tells them a story about the death of eight teenagers in Texas, and she says the counselor was involved. Are the girls making their observations fit their assumptions, or is something totally different going on? They receive a warning that they're in danger, but they ignore it. Bad move!

In the beginning, I felt like this was a silly mystery with girls having overactive imaginations, but it quickly became more serious. Although there was a main suspect, the author made me question the evidence. Then, I questioned it again. The resolution to the mystery was both expected and surprising. You'll need to read the book to see what I mean.