Hurting Distance (Spilling CID #2) by Sophie Hannah

Three years ago, something terrible happened to Naomi Jenkins - so terrible she never told anyone.

Now Naomi has another secret - her lover, unhappily married Robert Haworth. When Robert vanishes, Naomi knows he must have come to harm. But the police are less concerned, particularly when Robert's wife insists he is not missing.

Naomi is desperate. If she can't persuade the police that Robert is in danger, she'll convince them that he is a danger to others. 
Then they'll have to look for him - urgently.

Naomi knows how to descrive in detail the actions of a psychopath. All she needs to do is dig up her own traumatic past . . .