Dark Places, by Gillian Flynn

Libby Day's mother & 2 sisters were murdered at their home when she was 7 years old. Libby ran into the freezing snow and hid in a nearby field, lost some fingers and toes to frostbite, but survived. Her 15 year old brother, Ben, is blamed for the murders, and Libby's testimony helps put him behind bars for life. Now, 24 years later, after meeting a group that is trying to get Ben set free, Libby investigates what really happened that night, by interviewing people who were close to Ben, and even her estranged father. The club pays her for all her research into the case, which takes her all over Kansas & Missouri, as well visitations to Ben, who she hadn't seen since that tragic night. It turns out that Ben is innocent, but he is protecting someone else. The story keeps the reader guessing all the way through, and is almost impossible to put down for a break.