Bedding the Marquess by Kelsey Swanson

Gideon Bray, Marquess of Swanleigh, has always been a rake—a reckless, charming scoundrel never meant for happily-ever-afters. Yet, behind his careless smile lies a man scarred by neglect, and desperate for the family he was denied. Only one woman has ever seen past the mask: Caroline Wells, his dearest friend and the lone female in his notorious circle of comrades.
Cast out by both her family and Society after a youthful scandal, Caroline long ago learned to live by her own rules. Gideon stood by her, offering his loyalty when the rest of the world turned its back. But when one night of unbridled passion shatters this boundary between them, Gideon shocks them both with an offer of marriage, though neither of them knows quite how to transform years of friendship into the intimacy of husband and wife.