https://www.gutenberg.org/
https://librivox.org/
https://www.wikipedia.org/
https://www.loc.gov/collections/
I could not tell you a convenient way to download the entire archives of
these sites but they have free public domain books/audiobooks/etc that CAN be
mass-downloaded. You could archive these in trisquel, have a fetch program
which, everytime you connect to the internet will try and update. Therein is
the problem, I don't know if such a fetch program could be made because there
might not be consistent repositories for it to refresh from. If you are
willing you personally manually could do a annual update for each year maybe?
