Quick context: I'm making an asymmetric card game for two players, in which one player plays a spy stationed in a foreign country (e.g. the UK) during WW2, with the goal of gathering intelligence for his country. The other player's goal is to protect said secrets, and capture the spy. If you've played the game Netrunner, this game is partially inspired by that.
In that context, I'm curious about what methods spies would use during WW2 to gather intelligence, as well as how said intelligence was protected. I'd also be interested in any easily accessible literature that might provide me with further information on the topic.
Thanks in advance for any help!
Methods to collect primarily were recruiting sources who had access to the information the spy wanted. The source would meet with the agent at a time a location pre coordinated by code. Need a meeting? Perhaps moving a flowerpot to a window sill at a particular time. Need an emergency extraction? Call a number and say an innocuous phrase that had a certain meaning. Some had portable radios that could transmit a code. Counterintelligence relied on sources, basic security to notice suspicious information and picking up strange radio signals that you may not be able to understand but you hear and can triangulate. A common way for spies to move around was to travel to neutral countries like Switzerland that had enemy intel targets and you were somewhat protected by having a believable cover story and the host nation prevented outright hostilities. Unfortunately each case was unique depending on the target and location and mission requirements though. There are many excellent books about real life agents one doesn't need to bother with fictional stories. I liked the Great Betrayal myself, it focuses on WW2 and Cold War Intel operations and mole hunts from the British and Russian sides.