by Amleto Vespa, 1938 (I have a "Left Book Club Edition" -- which I thought was an odd name, like Left Luggage, until I realised it meant the political Left. Didn't realise Gollancz was a Commie).
Vespa worked as an intelligence agent in China for Chang Tso-Lin from around 1916. He spent his time tracking down arms smugglers and drug dealers, and a surprising number of white (sex) slave traders, whom he had a tendency to string up from the nearest telegraph post. In disgust at the pre-Fascist Italian leadership, Vespa became a Chinese citizen, which put him at the mercy of the Japanese as they started colonising Manchuria in the 20s and 30s. Chang was killed in 1928. In 1932, the Japanese forced Vespa to work for them, threatening to kill his wife and children in Harbin if he did not. So he worked for them until 1936, when he left, managing to extricate his family after the Japanese took all their wealth and Vespa threatened to write a book about his activities. He wasn't really a double agent, since he only managed to do a few anti-Japanese things, like get a conviction in one spectacular kidnapping. The catalog of Japanese rape, pillage and arrogance is long, though a few gentlemanly Japanese -- who tended to quit Manchuria as soon as possible -- are mentioned with respect. The book really only has one point to make, so is less interesting than it might have been, thought it seems unfair to make that comment, in the face of the humans who suffered.
Of course, if you really want to know about this interesting part of the world, the definitive study is by David. If only I had a signed copy!