Hi there and welcome
So we do not go over the steps you have already done, can you tell em what steps you have done to try to resolve this please?
This is usually an issue with the Firmware, so I would first like you to do this please.
Remove the charger from the Kindle and hold the power button to off for 35 seconds.
Release, then connect the charger and try to turn on the Kindle and see if the books download.
Let me know the results please