Sounds like a variation of the one-time pad. One affect all this does have is bringing back the days of having to use human agents to transport keys/code books around.
The beauty of the system I described is that it allows digital transmission. . . and it is still secure. You can also send the same message to multiple recipients so long as they all have the same book, rules, and calendar. Only the recipients can decode it. . . and since it never repeats for character, word, or apparent message, it is totally un-decypherable.