Nightfall - Stephen Leather (Jack Nightingale #1)

Goodreads 3.96

When he couldn't save a little girl who was being abused, policeman Jack Nightingale left the force and became a private investigator. He has recently inherited a mansion from a man who turns out to be his father. A father who killed himself when he was unable to reverse a deal he did with a demon - a deal that sold his son's soul.