Y is for Yesterday - Sue Grafton

Y is for Yesterday is the twenty-fifth in the Kinsey Millhone mystery series by Sue Grafton. The darkest and most disturbing case report from the files of Kinsey Millhone, Y begins in 1979, when four teenage boys from an elite private school sexually assault a fourteen-year-old classmate
and film the attack.
Not long after, the tape goes missing and the suspected thief, a fellow classmate, is murdered. In the investigation that follows, one boy turns in evidence for the state and two of his peers are convicted. But the ringleader escapes without a trace. Now, it's 1989 and one of the perpetrators, Fritz McCabe, has been released from prison. Moody, unrepentant, and angry, he is a virtual prisoner of his ever-watchful parents
until a copy of the missing tape arrives with a ransom demand. That's when the McCabes call Kinsey Millhone for help. As she is drawn into their family drama, she keeps a watchful eye on Fritz. But he's not the only one being haunted by the past. A vicious sociopath with a grudge against Millhone may be leaving traces of himself for her to find . . .