I really enjoyed this book! Like a hot bath at the end of a long day, a bowl full of mac-and-cheese, and a cold beer, this is solid comfort-reading for those who like m/m historicals.
This book is much more of a mystery than a romance, and it is set in England right before the start of the Napoleonic wars, and the plot focuses on tracking down a man who is passing information to the French. The two main characters are former lovers who split with unfinished business, and the book focuses on them solving the mystery as well as learning to trust each other and work together, and (naturally) coming back together as a couple. The sexual content is pretty light and the mystery content is more prominent, so this book is recommended to those who like a little less romance in their romance books and more action.
My only criticism would be that there's nothing too new or challenging here. The characters are a gruff working guy up against a suave, wise-cracking minor aristocrat who is more or less Fanon!Draco (for those old enough to have cut their m/m teeth in the Harry Potter fandom). And the dynamic here is a little well-trodden but it's well-trodden for a reason, it's a enjoyable romantic dynamic that works well for a lot of people.