I think all my suggestions are elsewhere in the thread, but I do want to point you to a couple sources for public domain books that are more readable than a lot of other sites.

https://standardebooks.org/ - Mostly cleans up and formats Project Gutenberg sources to make them more readable. Fixes typos, fixed formatting, properly uses ebook features. Very handy.

https://www.globalgreyebooks.com/index.html - Similarly cleans up and formats public domain sources to make them more readable, but has a lot of more obscure stuff. Especially good for weird bits of philosophy, theology, and esoteric stuff, but also just another good source to check if standard doesn't have what you're looking for.

I think someone else mentioned Anna's Archive, but that one's more than just public domain books. I've only used it a couple of times when I wanted to buy an ebook but the author/publisher set it to a price I wasn't okay with.