Dark Places by Gillian Flynn
Libby Day is the sole survivor of a family massacre that took place in 1985. Seven years of age at the time of the murders, her testimony clenches a life-sentence for her brother Ben, 15, who at the time was rumored to have been dabbling with Satanism. Libby, now in her early 30s, is in a financial fix. She becomes involved with a club whose members are interested in true crime and finds that none of the members believe her story. When they offer her cash for digging up her past and investigating key possible suspects, she reluctantly agrees and soon begins to question what she saw or didn't see on that cold and tragic January night.