Tang,
The good news is, while each e-reader tends to have their own preferred format you can stay fairly neutral with ePub format books and then migrate them specifically to your personal device if you use the Calibre software (OpenSource -
http://calibre-ebook.com/ ).
If you avoid getting DRM-encumbered books your life will be a lot easier (you can strip DRM out but it's more work).
Paul.