Check your local library for many WW2 historical accounts of the french resistance. Madam Fourcades secret war, Virginia Hill are a couple of my favorites.

Also memoirs from FBI and CIA agents, and anything on SOE, Churchills “Special Operations Executive” division which had as its objective to defeat the Germans covertly in Nazi occupied France, which he started because he wanted more results than the old school tie boys in traditional British intelligence.

For that matter accounts of the beginnings of the American CIA in WW2 are interesting too.

On all these, enough time has passed that there aren’t national security concerns any more, technology changes but the principles are always the same. Interesting reading and you can learn a lot, especially how networks are penetrated and how spies were caught.