One Marquis Murdered. The Murderous Miss Martingale, #4

Since meeting her husband Ruan, Baron St Trevone, Jemima has discovered a good deal more about the world of governmental intelligencers - that's spies to the uninitiated - than she had ever thought to learn. While Jemima has been disposing of ill-mannered men, life has been made all the more complicated by the sudden appearance - and disappearance - of St Trevone's former colleagues who find it difficult to believe he has given up the spying game.
And while it is true that Ruan is keen to withdraw from the murky world of espionage, there is one more job left to do. For long months he has been trying to track down a double agent, a man who thinks nothing of betraying his country and taking lives. He goes by the name l'ombre, a shadow that has eluded everyone for five long years. The discovery that l'ombre is one of his oldest friends is a bitter blow for he and Lord Christopher de Courtney have always been close.
Now Ruan is determined to end the man's career once and for all. Naturally, he won't be doing so without his devoted wife by his side. After all, Jemima knows exactly what to do with badly behaved gentlemen. It takes long weeks to track their quarry down to a house party in the snowy wilds of Staffordshire but less than twelve hours before Jemima discovers of a body in the library, one that she is assuredly not responsible for.
Now she and Ruan must unmask the killer, deal with a double agent and try not to cause their hostess any inconvenience. Nefarious business may be inescapable, but being considered a bad houseguest is simply inexcusable.