Thanks for sharing all of the interesting history. Yes, a book is not a one time pad, but in practice is probably more practical for a real spy in historical t

Thanks for sharing all of the interesting history. Yes, a book is not a one time pad, but in practice is probably more practical for a real spy in historical times, and probably "good enough." But the possibility of retroactively breaking all past communications if your book is seized is a huge problem. However, simply getting caught with a one time pad would essentially prove you were a spy. Nowadays of course, it is both easier to digitally conceal onetime data, e.g. on a regular phone, and easier to brute force a book based cipher, especially if you have access to large digital book archives.