The Amazon 'Free' section is dizzying, isn't it? You start to get book-blindness after a while... I love that it's a great place to snag some of those out-of-copyright books you've always wanted to read though - I got four of the "What Katy Did" books I read as a kid which was both surprising and nice. I think I saw some Jane Austens there too.
I'm not sure about other places you can buy Kindle books from - I know you can only download them wirelessly to Kindle from Amazon - but you might want to look up Project Gutenberg, which has free out-of-copyright books you can download and copy across to your Kindle with the USB cable. I've started trying to teach myself Latin with a book I found there.

Lots of things to choose from there too!