Take a look at Standard Ebooks[1] as well, which I heard from here at HN. Their motto is "Free and liberated ebooks, carefully produced for the true book lover", and they live up to that standard.

[1] https://standardebooks.org/ebooks

Text in various formats: https://libgen.li/ or https://www.gutenberg.org/

Audio books: https://librivox.org/

Libgen like Project Gutenberg often has poor formatting on eBooks. Librivox recordings are donated by volunteers. A single book might have multiple readers. Not ideal but you get what you pay for. Audible also has some classics for free, with subscription.

Kindle ebooks though they are hard to find by design, I think. Amazon wants you to buy stuff, even when the work is in the public domain. There is also, of course, Project Gutenberg [0], but I have not tried to interface with that project through ereaders. I wonder if anyone reading this has any additional tips :-)

[0] https://www.gutenberg.org/