No One Knows (DI Barney Mains #7)

Ten years ago a 13-year-old girl disappeared from her home.
And every single year her mother has begged the police to find her.
Finally she turns to private eye Barney Mains, a man who distrusts the police hierarchy as much as she does.
But then an ex-cop is murdered right after Barney's visit.
Is he being set up?
Is he leading the killer to his next victim, like some unwitting Angel of Death?