An Irresistible Alliance: Devil's Brood Trilogy -- Stephanie Laurens

<p><em>A duke's second son with no responsibilities and a lady starved of the excitement her soul craves join forces to unravel a deadly, potentially catastrophic threat to the realm - that only continues to grow.</em> </p><p>With his older brother's betrothal announced, Lord Michael Cynster is freed from the pressure of familial expectations. However, the allure of his previous hedonistic pursuits has paled. Then he learns of the mission his brother, Sebastian, and Lady Antonia Rawlings have been assisting with and volunteers to assist by hunting down the hoard of gunpowder now secreted somewhere in London. Michael sets out to trace the carters who transported the gunpowder from Kent to London. His quest leads him to the Hendon Shipping Company, where he discovers his sole source of information is the only daughter of Jack and Kit Hendon, Miss Cleome Hendon, who although a fetchingly attractive lady, firmly holds the reins of the office in her small hands. </p><p>Cleo has fought to achieve her position in the company. Initially, managing the office was a challenge, but she now conquers all in just a few hours a week. With her three brothers all adventuring in America, she's been driven to the realization that she craves adventure, too. When Michael Cynster walks in and asks about carters, Cleo's instincts leap. She wrings from him the full tale of his mission-and offers him a bargain. She will lead him to the carters he seeks if he agrees to include her as an equal partner