Buy other e-books and read them on your kindle
First, you need to download "Calibre" and use it to manage your e-books.
Then, you can buy e-books from sources other than Amazon, like Sony, Barnes&Noble, etc. They will be in different formats but no problem. Calibre will convert and upload them into your kindle.
Also you may download thousands of e-books for free (and legal) from guthenberg.org. These will be in .txt format but calibre will convert them as well.
I hope this helps.