A False Mirror by Charles Todd

Hampton Regis, a small harbor town on the southern coast of England, is a most unlikely place for violence. Yet, one spring morning, a man is found on the strand so severely beaten that he slips in and out of consciousness. The prime suspect? His wife's jilted lover, who served with Rutledge in the recently ended Great War—but who left the Front under a cloud. Badly wounded, yes, but did someone also cover up cowardice?Rutledge is called on to prove the innocence of a man he dislikes and distrusts. But the deadly triangle also stirs up memories of the woman Rutledge himself loved and lost when he went to France to fight. His doubts about the accused and himself only deepen when the victim of the beating mysteriously disappears, with no body to be found.As the brilliant yet tormented detective discovers that he's not the only person seeing a reflection of tumultuous emotions in this case, he must confront the demons that threaten to overwhelm him and search out the truth. For in Hampton Regis hides a vicious killer who intends to let nothing—and no one—stand in the way.

A love triangle turned deadly sends Scotland Yard Inspector Ian Rutledge to Hampton Regis, a small harbor town simmering with secrets. Accused of beating the husband of a woman who jilted him, Rutledges former trenchmate takes her hostage to escape arrest and demands Rutledge prove his innocence.