His at Night (The London Trilogy)

Lord Vere is used to baiting irresistible traps. As a secret agent for the government, he's tracked down some of the most devious criminals in London, all the while maintaining his cover as one of Society's most harmless--and idiotic--bachelors. But nothing can prepare him for the scandal of being ensnared by Elissande and forced into a marriage of convenience awash with seduction and dark secrets. Set in Victorian England.