Darkness and Light: (Frank Elder)

She wore a gold dress, short-sleeved, its skirt full-length and slightly flared. He could see the faint indentation on her left hand, a pale circle of skin giving away the fact that, until recently, a wedding ring had been there. She looked peaceful, lying there on the bed, her arms resting easily together, the left hand on the right, a slender silver cross and chain encircling her neck, and not a wrinkle, not a fold of her dress out of place. And, perhaps, she truly was at peace. For she was dead. This was the sight that greeted Detective Inspector Frank Elder on his first case with the Serious Crimes Unit. His first case and never solved; no one was ever charged; the murderer never found. At liberty to walk the streets, and to kill again. Eight years later, Elder's estranged wife contacts him in his Cornish hideaway. Her friend's sister Claire - a quiet and withdrawn widow in her fifties - has mysteriously disappeared. Elder, reluctantly, agrees to dig around and see what he can find. Then Claire is found, dead, arranged with meticulous detail on her bed, and it doesn't take long for Elder to make the connection.
It's obviously the work of the same unbalanced individual and, to find the killer, Elder must shine a light into the darkest recesses of human behaviour, the dark and twisted recesses of a disturbed human mind...