content: that is horrible security, very easy to break since all books are digital. Much better solution is do the same but write own book. You know, the code-book. Just use OTP with own page of codes. Separate page for person A and person B when they want to send a text (and other person have a copy of it obviously) - then if you can make sure you avoid re-use of code (e.g. sign it, confirm usage of 20 letters in some monitored but hard to spoof communication) and if you avoid using any electronic devices, then it is not crackable (ofcourse besides physical confiscation/getting copy of that paper).