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 

 I might give it a try again, I know it, the user experience was horrible and it does not solve the problem of sending articles to my reader on GrapheneOS from my computer's browser for reading with one click. I think the user experience is worst from all the readers I've tried.