The Heiress and the Spy — Julia Donner

Rich widow, Elizabeth Shelton, secretly yearns to break free from the confinements of her placid life. Her late husband died a hero in the Peninsular Wars, leaving her childless and badgered by his greedy parents. Leaping into love and a dangerous scheme to serve king and country had never entered her mind, but she's unable to resist the impoverished and dashing Lord Asterly when he asks her to become his wife and aid him in intrigue.