A Gentleman Fallen on Hard Times by Grace Burrowes

A man dwelling in darkness…
Lord Julian Caldicott has come home from the war in ragged health and with a reputation in tatters. All he wants is to recuperate in private without bringing any further scandal on the family’s good name. His godmother inveigles him into serving as her escort to a rural house party, whereupon circumstances conspire to prevent Julian from resuming his reclusive existence.

Must shine the light of truth on a dangerous deception.
Julian becomes the subject of rumors, lies, and whispers, and soon he’s faced with a choice: Wage one more battle for honor’s sake, or be branded a traitor to all that he holds dear. He will enlist unlikely allies, face old demons, and vex every other guest on the premises, but he will not stop until he’s brought a criminal to justice–or died trying.