Showing Last Night's Scandal by Loretta Chase

Peregrine Dalmay, Earl of Lisle, may have survived the deadly perils of Egypt, but back in Regency London, he faces the most dire threat yet: his irrational, emotional family...and the completely uncontrollable Miss Olivia Wingate-Carsington!

Descended from a line of notorious—but very aristocratic—adventurers, Olivia has a long history of driving Peregrine to distraction, and her debut into polite society hasn't lessened her flair for drama, or her ability to drag him into her scandalous schemes.

All Peregrine wants to do is escape back to his research and the lesser evils of poisonous snakes and tomb robbers, but his family has guilted him into an impossible mission in the Scottish wilds; and Olivia—who is keenly aware that a respectable future of marriage and rules and propriety looms—decides that accompanying him will be the perfect chance for one last adventure. Besides, she really, only wants to help.

Which is why Lisle and Olivia find themselves in a gloomy Scottish castle, inhabited by grumpy servants, spiteful ghosts, and craven murderers…and possibly the greatest peril of all: the wayward commands of their very unruly hearts!