Earl Crush by Alexandra Vasti

For three years, wallflower heiress Lydia Hope-Wallace has anonymously penned seditious pamphlets—and for almost as long, she’s corresponded with the reclusive Earl of Strathrannoch. When Arthur’s latest letter reveals his dire financial straits, Lydia sets out for Scotland to offer him the only salvation she can think of: a marriage of convenience. To, um, herself.
But the real earl has no idea who she is. When a bewitching stranger offers him her hand in marriage, Arthur Baird is stunned. And when he learns that his traitorous brother has been writing to her under Arthur’s name, he’s bloody furious. He’s content to live alone in his moldering castle, and he has no desire for a radical wife. (Or at least, he shouldn’t.)
But Arthur is desperate to track down his brother, who’s become dangerously entangled in British espionage, and he needs Lydia’s help. What he doesn’t need? The attraction that burns hotter each moment they spend together. As Lydia slips past his defenses and his brother’s mysterious past becomes a very present threat, Arthur will have to risk everything to keep her safe—even his heart.