Lord of the Privateers by Stephanie Laurens

The eldest of the Frobisher brothers and widely known as the lord of the privateers, Royd Frobisher expects to execute the final leg of the rescue mission his brothers have been pursuing. What he does not expect is to be pressured into taking his emotional nemesis, childhood sweetheart, ex-handfasted bride, and current business partner, Isobel Carmichael, with him. But is it Isobel doing the pressuring, or his own restless unfulfilled psyche?

Resolute, determined, and an all but unstoppable force of nature, Isobel has a mission of her own—find her cousin Katherine and bring her safely home. And if, along the way, she can rid herself of the lingering dreams of a life with Royd that still haunt her, well and good. Neither expects the shock that awaits them as they set sail aboard Royd’s ship, much less the new horizons that open before them as they call into London, then, armed with the necessary orders and all arrangements in place, embark on a full-scale rescue-assault on the mining compound buried in the jungle. Yet even with the support of his brothers and their ladies and, once rescued, all the ex-captives, Royd and Isobel discover that freeing the captives is only half the battle.

In order to identify and convict the backers behind the illicit enterprise—and protect the government from catastrophic destabilization—they must return to the ballrooms of the haut ton, and with the help of a small army of supporters, hunt the villains on their home ground. But having found each other again, having glimpsed the heaven that could be theirs again, how much are they willing to risk in the name of duty?