The Assassin and the Libertine by Lily Riley

The fate of France itself is at stake if these sworn enemies cannot change their ways—and their hearts. Daphne de Duras is a proper French duchess by day and fledgling assassin by night. Her latest mission is to dispatch justice and protect the French aristocracy by executing Étienne de Noailles, disgraced former noble, legendary rake, and vampire emissary to the court of King Louis XV. But Étienne’s alleged crime—the gruesome murder of Madame de Pompadour, the King’s mistress and Daphne's friend—doesn’t quite fit the dashing vampire’s nature. With his immortal days suddenly numbered, Étienne needs to convince his would-be executioner not only of his innocence, but that they should hunt the real killer together—a challenge almost as difficult as convincing himself that he isn’t falling for her. Daphne reluctantly agrees to a temporary partnership when Étienne persuades her that something more sinister is afoot. He can, after all, help her find answers in places she’s unable to go alone. And despite her deep loathing for any and all vampires, she can't help but start thinking of a few other places she'd like to go with him.