The Trouble With Rogues by Nadine Millard

Ophelia Delacourt has made it her life’s mission to teach poor, unsuspecting ladies about the dangers of rogues, rakes, and rapscallions. When one of her protéges faces ruin, Ophelia rushes to her rescue. And right into the path of one such troublesome rogue.
Caleb Feldman, Marquess of Guilford is growing bored of his hedonistic lifestyle. Not that he plans to change it any time soon. When he sees a mysterious, dark-haired siren floating through his party, for the first time in a long time, Caleb feels the excitement of the hunt.
Their encounter is not what either of them expects. And the life-changing consequences? Even less so. Ophelia will do anything to keep her secret but that might not be as easy as she hoped.

For the trouble with rogues is sometimes they’re just too hard to resist.