If you use the Amazon Kindle app on your Apple device, you should not install the 3.6.1 update for the app.

UPDATE: Amazon has released a 3.6.2 update and removed the warning about the previous edition. The new release includes “Fix for Registration Issue” and “Various Bug Fixes and Security Fixes”

AppNewser has more details:

Do you like reading eBooks on your iPhone or iPad using Amazon’s Kindle app? Then here’s a tip, don’t download the latest update 3.6.1. This advice is coming straight from Amazon. The company posted this note on the app’s listing in iTunes: “There is a known issue with this update. If you are an existing Kindle for iOS user, we recommend you do not install this update at this time.”