The changelog for the latest Kindle for iOS update notes that if you don’t update to the new version before upgrading your device’s OS, you might have to re-download all of your books from Amazon.
This is a fix for a Kindle issue with the upcoming iOS7 OS upgrade that may cause customers to have to re-register and re-download books from Amazon. Please download and install – this is a required update before upgrading to iOS7.
That’s another wrinkle in Amazon’s ongoing tug-of-war with Apple over control of the iOS e-book ecosystem. Amazon has avoided selling its books directly through the Kindle app on iOS because of App Store rules that would require the company to give 30% of the revenue from a sale over to Apple.
Apple is expected to announce the release date for iOS 7 at its media event on Tuesday. If past is prologue, iOS 7 should be arriving sometime during the week of September 16.