I have literally just resolved this by a full reset:
Have your Amazon login details ready.
In the settings menu on your Kindle, assign a password (anything will do, e.g. abcd - but remember it!).
Place the device in sleep (ie short press on power button), then press the power button to wake it. It will request the password.
Type in: resetmykindle
After a minute or two it will go through the factory reset process, including the WiFi set up and account details.
That's it! Go into your main Amazon account on a different device, find your books, check all you want and deliver to your kindle. Done!