If you give a rake a ruby

the long version

"Fallon, the Marchioness of Mystery, is a celebrated courtesan with her finger on the pulse of high society. She's adored by men, hated by their wives. No one knows anything about her past, and she plans to keep it that way. Warrick Fitzhugh will do anything to protect his compatriots in the Foreign Office, including seduce Fallon, who he thinks can lead him to the deadliest crime lord in London. He knows he's putting his life on the line. To Warrick's shock, Fallon is not who he thinks she is, and the secrets she's keeping are exactly what make her his heart's desire" -- from author's web page.