That's a legitimate complaint. I just paid the money to go premium to remove ads. I've been using it to read books since my Nokia 770 and it has everything I need from an ebook reader so I'm reluctant to change.
There's a fork of FBReader on F-Droid called simply 'Book Reader' which is being actively developed (last release 2019-02-08.) Might be worth trying out.
A good, basic ebook reader for Windows is FBReader. The Windows version hasn't been updated in years (development seems to have shifted mostly to Android) but it still works just fine.
There's a fork of FBReader on F-Droid called simply 'Book Reader' which is being actively developed (last release 2019-02-08.) Might be worth trying out.
Git repo is here: https://gitlab.com/axet/android-book-reader