I've just finished the book Double Cross by Ben Macintyre which is a history of the British efforts at counter espionage in WWII - particularly how they misdirected understanding of the invasion coming on D-Day. (Called Operation Fortitude) The book's name comes from the British propensity for puns - the efforts to disrupt German intelligence involved a committee of 20 (the Roman Numeral for 20 is XX). The efforts involved an unlikely group of people. Early in the war, with the help of some pretty sophisticated math and a machine called Enigma, the allies were able to break the codes used by the Germans (they also were able to do it with the Japanese). But this story is about how MI-5 was able to plant a group of individuals who became the German spy network in Britain. Thus, throughout the war leading up to D-Day they could spin stories that the Germans thought were true intelligence.
What was most amazing about the book was the audacity of the MI-5 agents. The leader of the counter espionage group thought at one point that they controlled the entire German network in the UK. MacIntyre weaves a fascinating tale - each of the five most important counter spies had odd characteristics and a tremendous amount of ego. Not all took money from MI5 for their work. One was a Polish patriot. One (pictured on the right) was a Spaniard who created an entire network of agents all from fantasy and then was able to sell it. He was so successful that near the end of the war, Adolph Hitler awarded him the Iron Cross. There were a couple of agents with questionable morals.
The book also deals with the lead up to D-Day and how one Lieutenant Clifton James (who had been an actor in civilian life) agreed to pose as Montgomery to throw the Germans off the place of the real invasion. The British were smart enough to understand that a Spanish diplomat with Nazi leanings would be good to see the fake Montgomery - so they arranged he was invited to an embassy, allegedly for a talk, but was able to see the fake Montgomery and then relay his impressions to the German high command. The conclusion about the Monty deception was that it may not have produced very much, except the amusement of the participants. The Brits were full of these kinds of deceptions.
This reads like a spy novel - but it is true.