Curiosity Killed the Duke by Emily E.K. Murdoch

Samuel Dellamore, Duke of Chantmarle, is on a mission . . . but it’s not the one everyone thinks.
Samuel has turned up in Edinburgh and convinced Society that he’s on the hunt for a wife. But his actual mission is to scour the city to discover the traitor spilling secrets to the French.

Samuel’s natural curiosity, good looks, and title have always been enough to get him what he wants—even when he’s not on the hunt for a spy—but a chance encounter with a Miss Lucy Finch shows him that she may be the exception to that rule. This mysterious woman quickly becomes a baffling and intriguing distraction from his real purpose in Scotland.
Unfortunately for Lucy.