Also, chances are your books will have Digital Rights Management. DRM is basically a lock on the book that means it can only be read on an ereader that has the right "key".
This "key" is different for each e-ink reader, tablet, or desktop program you use, so you can't even copy a book from "Kindle for PC" onto a Kindle Fire, unless you unlock the book, so...
In order to properly back up your books so that they can be read elsewhere, you will probably want to google "apprentice alf" and follow the directions you get there; unfortunately, it is against mobileread.com policy to give detailed instructions here. Good luck!
