Y Is For Yesterday by Sue Grafton

Sue Grafton is finally down to her twenty-fifth book in the alphabet series!  This book begins in 1979 and goes back and forth between 1979 and 1989.  Kinsey doesn't get involved until 1989 when she gets a call from a woman who has received a letter asking for $50.000 or the police will be given a videotape showing her son and several of his friends raping a female classmate ten years ago.  Lauren McCabe's son Fritz has just been released from prison where he served time for the murder of another female classmate, Sloan who was suspected of stealing the video.  The McCabe's want Kinsey to find the blackmailer and get the video back.
All those involved attended an upscale high school and were in line to receive scholarships at the end of the semester after taking the Proficiency Test.  Iris Lehmann steals the test and the answer sheet, so her friend Poppy Earl could pass and go to Vassar. This incident seems to be the catalyst for the everything that happens next.  Someone reported the test stolen so Poppy and her boyfriend, Troy Rademacher and Iris were all dismissed from school.  Sloan was blamed by Austin Brown as the person who reported the test stolen to the administration, so he told the group to shun her.  In return she "blackmails" Austin with the tape to stop the shunning.  This leads to her murder and Troy and Fritz are sent to prison.  Austin disappears and so does the tape. Now, the tape has resurfaced and someone is using it to blackmail the McCabes.
Long time landlord Henry has taken in two homeless individuals and their dog, which bothers Kinsey because she doesn't trust them.  Ned Lowe the serial killer from X has returned and is stalking her.  Between the blackmail case and finding Ned before he tries to kill her again, Kinsey has her hands full.  Great detective work, interesting story lines.