Yeah, I would think save in browser cache first and then sync periodically via nostr events. Maybe publish an event either every X minutes of if the user closes our of the app.

And this doesn't have to ne for books published on Nostr. Got to be mindful of copyright and not screw the authors over. But if you highlight something and make that public it could send that out as a quote.

The app could even end the quote with something like an Amazon affiliate link to jelp fund the app.