This. Also...

https://archive.org/

...has a mind-blowing array of free ebooks in their 'texts' subsection. One could probably spend many lifetimes reading there.

One other point: don't be bound to your region for buying ebooks from Big River. Get the right VPN (NordVPN is one that I know works for sure--a 'friend' told me :-)), connect to Big River US, buy and download what you need. This is the moral equivalent of DeDRM'ing an ebook that you paid for. They may not like it and try to block, but they're also benefiting from your business regardless, and I've never heard a case of them bugging an individual user about doing this. When I'm out of country, I use same approach to access my streaming media accounts--which I paid for in good ole US dollars, so I don't feel bad about accessing my account just because I'm somewhere else.