The Greatest Challenge Of Them All by Stephanie Laurens

A nobleman devoted to defending queen and country and a noblewoman wild enough to match his every step race to disrupt the plans of a malignant intelligence intent on shaking England to its very foundations.
Lord Drake Varisey, Marquess of Winchelsea, eldest son and heir of the Duke of Wolverstone, must foil a plot that threatens to shake the foundations of the realm, but the very last lady—nay, noblewoman—he needs assisting him is Lady Louisa Cynster, known throughout the ton as Lady Wild.

For the past nine years, Louisa has suspected that Drake might well be the ideal husband for her, even though he’s assiduous in avoiding her. But she’s now twenty-seven and enough is enough. She believes propinquity will reveal exactly what it is that lies between them, and what better opportunity to work closely with Drake than this latest mission with which he patently needs her help?
Unable to deny Louisa’s abilities or the value of her assistance and powerless to curb her willfulness, Drake is forced to grit his teeth and acquiesce to her sticking by his side if only to ensure her safety. But all too soon, his true feelings for her surface sufficiently for her, perspicacious as she is, to see through his denials, which she then interprets as a challenge.

Even while they gather information, tease out clues, increasingly desperately search for the missing gunpowder, and doggedly pursue the killer responsible for an ever-escalating tally of dead men, thrown together through the hours, he and she learn to trust and appreciate each other. And fed by constant exposure—and blatantly encouraged by her—their desires and hungers swell and grow…
As the barriers between them crumble, the attraction he has for so long restrained burgeons and balloons, until goaded by her near-death, it erupts, and he seizes her—only to be seized in return.

Linked irrevocably and with their wills melded and merged by passion’s fire, with time running out and the evil mastermind’s deadline looming, together, they focus their considerable talents and make one last push to learn the critical truths—to find the gunpowder and unmask the villain behind this far-reaching plot.

Only to discover that they have significantly less time than they’d thought, that the villain’s target is even more crucially fundamental to the realm than they’d imagined, and it’s going to take all that Drake is—as well as all that Louisa as Lady Wild can bring to bear—to defuse the threat, capture the villain, and make all safe and right again.

As they race to the ultimate confrontation, the future of all England rests on their shoulders.