I'm looking for recommendations. I usually use Overdrive with books I've downloaded (I think in the epub format). But now I hear that Overdrive is no longer being supported (I think the company wants people to use their Libby app instead). I'm hoping to find something open source and privacy minded. Also not wild about what Overdrive is doing to public libraries
If you're on Android, I'm a fan of Moon Reader (+ version to support dev). It'll cover pretty much any reading file including comics. That's if you already have the files.
Local library is probably the best and safest route for getting ebooks. There are libraries online that archive books as part of their mission statement. Legality is grey, but there's also archive.org and project gutenberg for books outside of copyright.