Because of lack of altenatives I decided to write a small ebook reader plugin for Nextcloud. The task of creating such an app was quite easy: I just forked the files_pdfviewer extension and replaced PDF.js with the Epub.js library. In the app template file, I used the reference ebook reader interface which is good enough for simple preview purposes.
Still this interface might lack some features compared to desktop ebook reader apps like the Gnome GTK-tool Foliate, so I’m still looking for better implementations of Epub.js.
You can find the ebook reader app in the Nextcloud app store and the source code or issue tracker in our public Gitlab repository.
