Where Have All the Scoundrels Gone by Louisa Darling

The cold, inflexible Duke of Ashbourn has a secret: he spends his nights taking on all comers at the Nemesis, a no-holds-barred underground boxing ring. The brutal fights are his one outlet, allowing him to maintain ruthless control of his life…until he meets Elizabeth Pickford.
Bess would do anything to aid her friends, including chaperoning Ashbourn’s half-sister. Steady and practical, Bess longs for passion and is instantly drawn to the mysterious duke, though she believes he could never desire a woman so far below his station.

Consumed with his need for Bess, Ashbourn vows not to allow his darkness to touch her…but Bess’s search for a scoundrel brings her to the Nemesis. Masked and nameless, they surrender to a forbidden intimacy too seductive to be denied. And when their secrets are bared, they must risk it all to claim one another in the light of day.