My favorite example is a Spanish writer who sold fiction to the Nazis during WW2 in the guise of espionage. He claimed to be writing reports of shipping and troop movements from England when he never left Spain.
By reading English newspapers, plus studying maps, railroad timetables and guidebooks he made some very shrewd guesses.
When his spy masters asked where he was getting his information he told them the English sailors and dock workers were a demoralized lot, and you could get all the information you wanted for the price of a liter of wine in the many bodegas lining the Portsmouth waterfront .
This is just plain fascinating. Egads. So many wonderful aspects of WW2! (And lots and lots of ideas to spark my own writing!)