Good one, Jude.
Secret and Urgent
Fletcher Pratt, on ThesauruS
This is a historical story of codes and ciphers, what was invented when, how they work, how to decipher them, etc. It's way out of date for modern stuff (I read it when I was growing up, and snagged my parents' copy from the estate) but if you want to know something like how Napoleon's spies enciphered their messages and who figured out how to read them, it's pretty entertaining.