i have the 900 too. I have also found many book are not available as Epub that are older books.
Check the following places for books
1. Sony's books web site
2. Under Sony books, click on Google books on the screen on the right hand side.
3. Booksonboard.com is a good place to download books that will work with your E Reader.
There are conversion things you would have to do at Amazon that are a pain. I would suggest sticking to the 3 options I gave.