A Duke Is Always Dangerous by Rebecca Leigh

After living a rather boring life as the daughter of a small village vicar, Abigail Bailey longed for adventure. When her best friend left for London a year earlier, she had warned her friend of the dangers lurking around the city. But when her friend began writing letters to her detailing the excitement and thrills she was experiencing while moving about the ton during the London season, Abigail began to dream that one day she would be able to have adventures of her own. After the death of her father, Abigail was invited to join her friend Emma and her new husband the Duke of Sunbridge at their Scottish residence, Castle McDonough. While traveling to McDonough Castle she inadvertently crosses paths with ex-spy Camden Davenport, Duke of Stafford. Camden was handsome, mysterious, and unlike any man she had ever met before, but she was not prepared for the risky escapade they were about to embark upon, and it makes her question whether she has the strength to endure it, not to mention the power to withstand the charms of her new guardian.
When Miss Bailey overhears a plot that puts both of their lives in danger, Cam finds the attractive young lady thrust upon him, and in order to keep her safe, they must travel from Scotland to London on a journey that will test both of their limits. Cam’s attraction to Abigail only grows stronger the more time he is with her and when a dangerous villain from his past threatens them both, he realizes how far he will go to keep her safe.