The Lair of the Burrells (The Brazen Burrells #0.5)
Lynne Connolly

When their ship is wrecked on the coast of Scotland, Lady Cecilia Burrell finds herself the guest of her husband's grandfather. He's an impoverished, unwelcoming grouch. She's put into a cold bedroom and left to herself, even though she is bearing the heir to the old man's title. She doesn't know if Frank is alive or dead. And there's a ghost wandering around the house.
Frank finds himself on a secluded bay, injured and freezing cold. A war hero who'd gone through the horrors of Waterloo, he never thought he'd die this way. Only the help of one of his grandfather's tenants gets him to his grandfather's castle, where he finds his wife distressed and abandoned.
They need to get back to civilization while they still can. But something is very wrong in this place, and Frank, as the heir, needs to resolve it.
(Note: This story was first published as part of the USA Today Bestselling Boxed set Upon a Midnight Dreary, October 2021)