I'm writing a novel that involves an extensive underground intelligence network, set back in the day. I'd like to style it after late feudal france, if that helps. Is there anywhere I can read extensively about spies, how they communicated, maybe some examples of assassins at work, maybe even what the average spy did during his day-to-day activities? I'd help a lot. Thank you.
The Code Breakers by Simon Khan has a great deal of information on the black chambers of Europe. These details are generally focused on the interception, secret opening and reading of mail, especially enciphered letters.
Might be a good starting point.