Ubuntu – Failing to unrar files

I'm sorry to ask this, but all of the questions along similar lines I could find have been answered by installing unrar/7zip.

So, I received a book in the form of a .rar archive of jpegs. Initially, I tried to uncompress with 7zip from terminal. It goes through each file in the archive and for each file in the archive, says,

Extracting   cover.jpg     Unsupported Method

After doing some quick googling, I thought it was due to lack of .rar codecs, as it's not technically freeware or somesuch.
After installing unrar, from the terminal I run

unrar -x Books.rar

And it goes through and just says "Failed" in the place of 7zip's "Unsupported Method."

I thought it could be a corrupt file or something, so I had a friend compress a random jpg as .rar and send it to me, and I encountered the same program.
Double clicking it opens unroller, which then promptly closes.

Any help would be much appreciated.