Like The Stranger Beside Me, written by Ann Rule, who realised while writing the book that the wanted serial killer she was trying to track down was in fact her friend and former colleague, Ted Bundy.
Maybe it was The Story of Mary Bell, about an 11-year-old girl who murdered two young boys. As an adult, after being released, she agreed to be interviewed by Gitta Sereny to give a previously untold backstory into her childhood and the reasons she may have committed such awful crimes.
Or maybe it was Columbine, about the 1999 High School massacre.
Have you read any true crime books that were utterly gripping?