Currently I’m using booky mcbookface but the app is really lacking and the scrolling is not smooth.
Edit: I came across Lithium and Readera which both don’t seem to collect much information compared to other apps. Not sure if they’re open source though.
I use Librera on Android. I generally convert to .txt when I read fiction on Linux because I can use a wide range of text editors/viewers that way. It has been a great way to familiarize myself with a lot of features that I don’t use when I’m tweaking config files.
Beyond that, I use Okular or Calibre’s reader for epubs on an as-needed basis.