The Kindle has enough storage space to carry way more books than you could ever read in year. You can load hundreds of books into the Kindle memory. As far as connecting it through a mobile phone or tablet, you would only be able to side load non-DRMed mobi books. You would not be able to buy DRM protected Kindle books via the phone then transfer them to the Kindle via the USB connection with the phone. You could transfer PDFs which in many cases look horrible on a Kindle and you could of course transfer non-DRMed books. You can also buy Kindle books from any device and Whispersync then to any other device, but that requires a WiFi connection.

IMHO, it would be easier and possibly less expensive to buy a Kindle with 3G radio built in and circumvent the mobile phone kluge connection all together.