Darkness & Light — John Harvey

Frank Elder, shut up at his hideaway in Cornwall, receives a call from his ex-wife, Joanne. Her friend Jennie, a brisk businesswoman, is concerned for her sister, Claire, who seems to have gone missing. As there are no signs of foul play, the police have categorized it as a low priority. Will Elder come and investigate? Reluctantly, he agrees.
Claire's life was not as quiet as her sister had assumed. It doesn't take Elder long to find out that she has had a number of sexual relationships with men whom she met over the Internet. But before Elder's inquiries have got very far, Jennie calls him with the news that she has found Claire's body, fully and carefully dressed and laid out on her bed. Despite having no apparent wounds or other signs of trauma, she is clearly dead.

Elder immediately thinks back to 1997 and a crime all too similar to this one. The murderer was never found. Taken on as a civilian consultant, he reopens the cold case but as he investigates further, he makes the chilling realization that there may be two killers out there; one who's tried to rehabilitate himself; the other poised to strike again.