The Marquess and the Maiden by Robyn DeHart

Harriet Wheatley is the mastermind behind the Ladies of Virtue’s quest to rehabilitate the gentlemen of the ton. So when it comes to selecting her own target, she knows who to choose: Oliver Weeks, Marquess of Davenport—the most extravagant wastrel in all of London.
Known for his opulent lifestyle, Oliver isn’t surprised by Harriet’s confrontation. It’s obvious Harriet longs for any way to take him down a notch. She says she’ll help him find a bride, but he knows it’s only because it gives her the excuse to chastise his indulgent ways.

Oliver has good reason for his flagrant overspending, but Harriet will hear nothing of it. So he has no choice but to teach the lady a lesson, even if it means risking his heart to the hard-headed and fiery woman.