yEnc isn't a cipher, but rather an encoding for mapping binary to text, similar to base64 (but much more effective). So this denotes yEncc encoding.

The files you're seeing are PAR2 files, which are used for repairing. They're useless without the base file. The file in your example contains 32 recovery blocks. That means if your base file has 32 or less damaged blocks, this parity file can repair it.

Usually, you'd download all files belonging together in a single download and let your downloader do the rest. This is normally done by loading an NZB file that you either get from a Usenet search engine or an indexer.

Sounds like there's a step missing somewhere.

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.

You need better (e.g. private) indexer 😇