I think 'Librera" is the best open source eBook reader app for Android. It's available through the F-Droid app store.

As for Linux, I don't know for certain I just use the generic pdf reader that comes with my distro.

I hear Foliate is a good Linux eBook reader, and Linux Mint recently announced "Thingy" which sounds corny, but reviewers say it's pretty good.

 https://github.com/linuxmint/thingy