Question about Kindle and Yesterday's Classics

I need some help and thought someone here could possibly help.  I had ordered the books from Yesterday’s Classics for the kindle about a year ago.  I think I saved the files on my portable hard drive.  I can’t really remember how I got them to my kindle, but they are all there on my kindle– no problems.  My portable hard drive doesn’t work anymore.  I am worried that I don’t have the Yesterday’s Classics books saved anywhere.  If my kindle were to die, would I be able to get the books on another kindle?  So I looked at my amazon kindle account and it has a library of all my kindle books that I got from Amazon, but none of those from Yesterday’s Classics.  I can’t figure out how to get the Yesterday’s Classics books on that list.  Or how do I save the books so I won’t lose them if something happens to my kindle?  

I plugged my kindle into my computer with the USB cord and tried to save the kindle files to my computer, but when I try to open them, they don’t open.  If anyone understands all this and can give advice, I would appreciate it.

Yolie