You should probably use a downloader such as NZBget or SABnzbd to download the whole collection of .rar archives (if it’s archived at all) and .par2 files associated with it. You’d download an NZB file for which content you wish to have, which is a simple file that tells the downloader how to download everything, fix them up when necessary and to repair the files, to finally extract a usable file for you.
That output is something Readarr should be able to use, like an epub.