All Good Dukes Come to an End by Emily E K Murdoch

Adam Seymour, Duke of Gilroyd, make a vow and he’s not going to break it. Probably.
After all, when he made that vow never to marry, it was in the depths of grief. Losing his wife so suddenly had never been the plan—and so throwing himself into serving the Crown was the obvious choice to forget the pain.
Which was easy, until Mr. Smee forced a new partner on him: Yates.
And not a Mr. Yates, either. No, the woman with the striking blonde hair and mischievous way with knives couldn’t be described as a Mr. Yates…
Dottie Yates isn’t wholly enamoured with Adam either, but she needs a man to pretend to be her husband. Preferably one with good connections. A duke will do.
Thrust into a fake marriage, spying in Brighton, and misunderstandings that threaten to reveal secrets of their past, neither Adam nor Dottie are ready to give up hunting down the Glasshand Gang—or the traitor who is apparently in their midst.
But as tensions heat and passions rise, it’s easy to forget spies, traitors, and vows when faced with growing love. A love that could end their growing friendship—and their mission.

After all, all good dukes come to an end…