A Deal With a Notorious Devil by Aydra Richards

Miss Phoebe Toogood has never wanted children of her own and, at nine and twenty, has spent a decade avoiding matrimony in her efforts to remain happily childless. But now that her potential suitors have turned toward widowers seeking mothers for their children, Phoebe is growing increasingly desperate.
The bastard son of an earl, Christopher Moore has been both criminal and spy in his time. Now that his connection to his half-sister, Lady Emma, is public knowledge, he knows he must polish his dreadful reputation lest it taint hers, too. For that he needs a damned wife. Not just any wife, but a proper lady from a respectable family to ease him into a society that will not accept him otherwise.
When Phoebe’s family moves into the house next door to Chris’, they forge a secret friendship between them. Neither wants to be married, but they need each other nonetheless—so they strike the perfect deal. A marriage of convenience, to the benefit of both. At least until their convenient marriage becomes somewhat less than convenient…