I have tried all sorts, but I cannot get back the unread books.
Go into your Amazon account then content and devices select the books you want and send them to your device.
UNLESS you mean get rid of them in which case it is the same except delete. If you do delete them they have gone forever unless you buy them again.
If I have helped you with an answer please click "Was this answer helpful" If not post and have a moan and I will try again if I can.
Believe nothing you hear and only half what you see.
If you need to contact Amazon use this link CONTACT AMAZON
This prompts the time to read the really useful User Manual on your device so you will be able to easily understand what is going on and how to get the best from your device.