I can't help thinking that these books might have been keys for encrypting messages. Key-based substitution ciphers, such as the Vigenère cipher[1], were already known in the 1500s. If two people each had a copy of this book, they could send a key to each other by referring to a sequence of characters such as "the third row on page 25". The messenger who carried this message wouldn't be able to reconstruct the key without a copy of the book. If you wanted to give the code to somebody else, you wouldn't have to send them the book: just tell them the "seed" words and the algorithm for generating the pages, which could be sent separately, for security.
Using this as a keypad (modular addition) would be much better than the Vigenère cipher, since there aren't such obvious correlations between key characters-- a good cellular automata is closer to a proper PRF.
[1] https://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher