I finally got rid of those lingering, "returned" library books that remained months after I'd deleting them from "Manage Your Kindle" on Amazon.
You have to Deregister--and then Re-Register--your iPad. This will delete everything from your device, but you can easily download all the books you own, stored on Amazon Cloud, back onto your device.
1. Go to iPad settings.
2. Click on Registration/Update.
3. Click "Deregister this Kindle." You will get the message, "Deregister will remove all downloaded content from this Kindle, do you want to continue?"
4. Click "OK."
You will land back on the kindle page.
Sign into your Amazon account, and all the books that appear in the Cloud will be available. Press the downward arrow on the bottom right of each book icon to download onto device.
This worked for me.
Good Luck.