I upgraded my first generation iPad to 5.1.1, my Kindle app books "appeared" to all come back. However, I get the error message, "Please remove the book from your device and redownload it from your cloud items. If the problem persists, try to deregister your application and register it again." I have no idea what they are talking about. I do not know how to remove the book, nor do I know how to deregister my application.
To remove the book, go to the Kindle app home page, select the book desired and swipe from right to left. A red archive button will appear, tap it and the book is removed from the app. To download it again, go to cloud view and tap the down facing arrow that is located where the archive button was. To deregister the Kindle app, tap the settings icon on the bottom right of the Kindle home page, tap registration, then tap deregister this Kindle. To re-register the Kindle, go into the settings like you did to deregister it, the button will now sat register this Kindle, tap it and follow the directions to re-register your Kindle app.