Damnation Street (Weiss & Bishop #3) by Andrew Klavan

They are two sworn enemies with a single obsession: a woman on the run from them both. Scott Weiss is a private detective. John Foy is a professional killer known as the Shadowman. The woman is Julie Wyant, an ex-prostitute with a hidden past and uncertain future.

Julie spent one night with Foy - a night of psychopathic cruelty that Foy called love. Desperate to get away from him, she vanished without trace, but Foy wants her back and won't rest until he has her. The only man who can find her is Scott Weiss, a middle-aged private eye with a romantic soul and an empath's ability to read others. Before she disappeared Julie begged him not to come after her, fearing that he would bring the killer in his wake. But Weiss can't stay away, believing that the only way that Julie will ever escape is to force a final confrontation with the Shadowman...