I am guessing you are saying this because in the latest update they removed the "store" button. If you read in the notes this was a requirement Apple made for them to keep the app in the App Store. You can go on their website and buy books and they will load to your iDevice. If you go to their website on a computer it talks about creating a link on your phone to buy a book on safari on your device.
"New Features on the Kindle for iPhone App
This update removes the Kindle Store button from the app. Customers can shop for 950,000 books in the Kindle Store by visiting Amazon.com/kindlestore in Safari or any web browser.
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