Duchess by Day, Mistress by Night by Stacy Reid

Georgiana Rutherford, the Duchess of Hardcastle, seemingly has it all—wealth, pedigree, and the admiration of the ton, except her heart hungers for a passionate affair. She meets the enigmatic and ruthless Mr. Rhys Tremayne, a man known to low and high society as the Broker. The attraction between them is impossible to deny, but she cannot be feeling it for this man.

Rhys Tremayne has built his wealth and empire by dealing secrets on the black market of the London underworld. He is determined to take his sisters away from the depraved world they’ve known their entire lives, and the duchess is the perfect woman to help sponsor his sisters into society. The only problem is that he wants more from Georgiana, even if the social divide between them ensures she can only ever be his lover in secret.