X for Extortion: 14 Manchester Square (The Seventh of December #2)
Garrick Jones

However, along with items Tommy has recovered for the duke, he has also discovered a secret stash of documents, which, when opened, prove to be a poisoned chalice. Tommy and Shorty find themselves caught up in a dangerous web of lies, enemy agents, assassins, and traitors. In an effort to save the reputations of not only their friends, but men and women high in both society and in the government, they themselves become victims of I.K.S., a former World War One international extortion ring, which has risen, phoenix-like, from the ashes of bomb-devastated London.