The Deserted Heart by Mary Lancaster

Unmarriageable? Or simply unusual?
There can only be one reason for the proud Duke of Alvan’s proposed visit to Audley Park. He means to offer for Lord Overton’s beautiful daughter, Thomasina, thus saving the family’s waning fortune. In the midst of the hectic preparations for his arrival, Overton’s least marriageable daughter Charlotte remembers to collect her young brothers from school for the holidays.
When fog forces them to spend the night at the Hart Inn, they are astonished to find the house deserted, save for one other enigmatic traveler who deals most capably with armed intruders. Drawn to their unconventional new friend, Charlotte enlists his help to solve the mystery.
Amidst the upheaval of the duke’s visit, to say nothing of the chaos caused by Charlotte’s unmanageable pet terrier, the Hart becomes the focus of nefarious doings, kidnappings, and romantic entanglements. For Charlotte is unwise enough to fall hopelessly in love with her sister’s intended husband, and the duke hides too many secrets of his own.
Will they find their happily ever after with each other?