...And Never Let Her Go, by Ann Rule

This is the true story of the murder of Anne Marie Fahey on June 27, 1996. She was secretary to the governor of Delaware, and disappeared that June 27th after a dinner out with her secret lover and prominent millionaire attorney & former state prosecutor, Thomas Capano. Capano was then separated from his wife, but had also been secretly seeing another woman named Debbie for 18 years. Thomas Capano was a seemingly likable, quiet family man, but after weeks & months of investigation, which included Anne Marie's diary entries, emails and interviews with countless people, it was discovered that Capano had another side: a manipulator driven by power and greed. It took three years to uncover the truth behind Anne Marie's disappearance and death, her family waiting the entire time to find out where their sister was. Capano had disposed of her body far out into the Atlantic Ocean, and her remains were never found. Capano was sentenced to death by lethal injection, but several years later after a lengthy appeal, his sentence has been changed to life in prison without possibility of parole (per internet).