I use librera reader and I have no ads, are you sure you're using the F-Droid version of the app? If you're not, I suggest installing it through F-Droid, as the play store version may have proprietary code in it. Many apps have different builds for Play Store and F-Droid, with slightly different featuresets.

If you are already using the F-Droid version, it should be reported. The list of anti-features is kept accurate by user reports like yours.

https://gitlab.com/fdroid/fdroiddata/-/issues