Originally Posted by dwig
your K3 seems to no longer be "registered" to the Kindle account from which the books were purchased and the books' DRM is preventing them from being read. Simply re-registering the K3 should heal the problem.
This shouldn't matter. You can de-register a Kindle and still be able to view all content that was originally on it. The DRM is tied to the device at the time the content is downloaded, not to the account.
Try a Factory Reset.