The Secret Adversary
by Agatha Christie
Guardian 1000 Novels
While The Mysterious Affair at Styles is a prototypical mystery novel, The Secret Adversary is really a spy story, with Bond-esque stakes involved. If amateur adventurers Tommy and Tuppence can't find a girl and a document, all of Western Civilization could collapse.
I suspected Julius early on because he used some British slang - "flat," "lift," etc. Is that how Americans talked in that time period, or was Christie taking some liberties?