The Sinner — C.J. Archer

Lies, desperation and an offer she can't refuse send impoverished widow Lady Catherine (Cat) headlong into marriage with the man who killed her husband. Guilt and desire battle within Hughe, Lord Oxley's, soul. When the enigmatic leader of the Assassin's Guild learns that the widow of one of his targets is at the mercy of a cruel man, he does the only thing left in his power to do. He marries her. Then the worst thing happens. He falls in love with her.With more than one person trying to kill him and a family to rescue, Hughe needs the help of all his Assassins Guild friends to stay alive and keep his wife from learning the truth. Because he knows, and dreads, what will happen when she discovers what he did.