The Hellion’s Secret by Jess Michaels

Phineas Blanchard, the Earl of Delacourt, feels restless and he can’t place why. It’s like there’s some itch he can’t scratch as he goes through the motions of his duties to his title and the dissipated pleasures he finds as a rake in hells all over London. Until, that is, he encounters a mysterious woman one night. One who bewitches and challenges him in ways that keep him dreaming of her. Imagine his surprise when he later realizes she is the infamous masked female boxing champion only known as The Hellion.

Esme Crawford is the daughter of a marquess and ran from her life in Society when her father died under mysterious circumstances, leaving her in grave danger. Becoming The Hellion was a way to survive and retake control of her existence. But now Finn is dancing around the edges of her deceptions and if she plays too close to his fire, she will surely be revealed and possibly burned.
Neither can stay away from the other and eventually Finn discovers both her true identity and the fact that she believes her father was murdered. Now a passionate affair rages between them, and a treacherous investigation brings them closer and closer to a man more than willing to kill. Will they be able to overcome the scandal of their passion is one question. Will they survive is quite another.

The second book in the About An Earl series, but it can be read standalone.