Comment 4 for bug 1584522

I think that until GNOME Books doesn't support .epub files, there's not a big gain in displaying it. I think Evince should be used to open .djvu files, as the DJVU file format is usually used for compressed scanned books (or that's my experience, at least), so from a UX point of view, it should be handled by the same program that handles PDFs.

I personally do use Books for its amazing features regarding collections (inherited from Documents, of course), but as you said, it's not a good idea to have the app if it shows files it can't open.

Hopefully, this will be fixed soon when native ePub/MOBI support comes to GNOME Books.