Hello. Thank you for coming to JustAnswer.
Sorry for the wait. My name is Michael. I will be helping you with your question today.
Don't worry about your books - you will not lose those as they are associated with your Amazon account.
The first thing I would do is a reset of the device if you have not already. This will clear the device and take you through the setup again. Once it goes through the setup, it will resynchronize all your purchases.
Even if you were to replace the Kindle with a newer version, your purchases would resynchronize.