pdfs of the tripcode books
some research on book ciphers:
https://www.wondersandmarvels.com/2012/11/whats-in-a-book-a-brief-history-of-book-ciphers.html
https://mysteriouswritings.com/mw-codes-ciphers-and-puzzle-series-the-ottendorf-cipher/
https://en.wikipedia.org/wiki/Book_cipher