My last trip home, dad treated us to a family viewing of the fantastic tail of intrigue -- James Bond: The True Story. It is the story of Dušan Popov, who also plays a large roll in this fabulous gem of WWII history and spycraft:
Double Cross: The True Story of the D-Day Spies
A thoroughly entertaining history lesson on the incredible deception that made Operation Overlord possible. Operation Fortitude was years in the making, and relied on perhaps the most improbable group of characters possible. But somehow it held together, and Germany was tricked into waiting 7 weeks before sending reinforcements to Normandy.
(somewhat spoilerish. But I assume you already know who wins in the end)
-- An astonishing number of double agents did not have to be 'turned', they allowed themselves to be recruited as spies for Germany and then immediately went to the Allies, to turn themselves in and work against the Germans
-- For the most part, the double agents weren't British or German -- they were Spanish, French, Polish, Slavic, Russian... Also for the most part, they were completely unqualified for spy-work, were filled with delusions of grandeur, erratic... keeping them happy and out of trouble was a herculean task
-- Several agents became so trusted that Germany passed on their messages verbatim, which allowed the British (intercepting messages) to have a quick cheat-sheet to code breaking all the other radio traffic of the day
-- Apparently the Germans had an incredibly strange confidence in the idea that one could just parachute unprepared, untrained foreign spies into England and these agents would somehow land safely, unnoticed, and could just insert themselves into the English country life.
-- Many Germans who were anti-Nazi or otherwise just did not want to fight were able to find their way into the Abwehr (military intelligence/spy service). Until it was routed and folded into stricter German departments they could travel pretty freely, and often wound up in offices like France, Spain, Portugal, where they could work off-book and, for a time, obey their conscience as far away from central Germany as possible. Many of the plots to assassinate Hitler came out of the Abwehr.
-- Along with the conscientious objectors, the Abwehr also had a lot of lazy, corrupt officers, resulting in a pretty ineffective group overall -- these officers were so fearful that they would be swept up in retribution if one of the spies did not turn out well that they covered over all sorts of obvious signs their agents were actually doubles
Radio transmitters, Secret Ink, Coded telegrams, Parachute Drops, Kidnappings... the stuff of Old School Spy Craft. But it also explained a lot of the Bureaucratic and Human Elements -- the infighting, plotting, corruption, and outright mistakes -- that make war planning such a messy thing.