Hi, thanks for using JustAnswer.com I am Cris and will be glad to assist you.
This appears to be a glitch within the software and can be resolved. Press and hold the power button for 20 seconds for the device to shut off then turn it back. Wait for the device to fully boot up and restore connections. Check to see if the books have now downloaded.
Let me know the outcome, reply back to me if the solution does not work. If it does please rate my service at the top and submit.