Weekend hacking after @topher1kenobe pointed me at “Ottendorf Cyphers”.
I’ve heard about these, though I didn’t remember the name.
It’s a cypher/code where two people have the same book and the code is page/word/letter.
Topher tried to do this with web pages once. I made my own version with bookmarklets.
https://page-cypher.veryuseful.app/
(My original attempt at building something failed because CORS!!!! 😬)