Secret Diplomacy: Espionage and Cryptography 1500-1815
by James Westfall Thompson and Saul K. Padover
It is fascinating stuff! Can you imagine trying to break the cypher on a medieval document that could be in Latin, French, Italian, or who know what language? And there is enough raw material for more than a few great novels and movies in here.
In fact, I am reading it as research for a book I'm writing someday. (There are two others I want to write first, but I want to set them all in the same era, so I need to pick the right era.) I already have my plot, but maybe I'll find a subplot or two....