An Irresistible Alliance by Stephanie Laurens

 Michael Cynster is startled and relieved at the beginning of this book to realize that, with the engagement of his older brother, he is no longer going to be as big a catch on the Marriage Mart. He can relax. But a hedonistic lifestyle hold no appeal; he's done that for awhile. Instead, he focuses on helping his brother with uncovering a plot involving some gun powder. Gun powder that is being moved by carts but none of them understand how shipping works. But Cleome Hendon does.
Cleo thought that working as her family's business as the manager would be exciting. And it was... for awhile. No, she's got it all figured out. So when Michael comes to her with a built-in adventure, she invites herself along. 
An okay story. Some light contortions to get a lot of people from previous books involved.

Three stars

Follows The Lady By His Side

Followed by The Greatest Challenge of Them All

This book came out March 11th, 2017

Opinions are my own