Hi there and welcome
First as a test, if you click on my link below and sign into your Amazon account, can you see the missing books there?
Please let me know
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 download the books.
Let me know the results please