The Lady Plays Her Ace (The Langley Sisters) by Vella, Wendy

Oliver (Ace) Dillinger is a man who has made his fortune the hard way, first with his fists, then by learning to be both a gentleman and a businessman, before entering the fringes of London society. He is ruthless, determined and trusts only a select few. Only one person unsettles his world, Lady Althea Ryder. She is feisty, opinionated and beautiful, and as a duke's daughter, far above his reach. Determined to put distance between them, he pushes her away, but Thea pushes right back, not willing to be set aside and Ace soon realizes that walking away from her is going to be the hardest challenge he has ever faced.