SOLVED: Not seeing books on Kindle Fire HD
EDIT: I found the solution to my problem while writing the post. I'm leaving the description below in case it helps anyone else.
SOLUTION: The Kindle Fire HD doesn't seem to support books in subfolders of the "Books" folder on the device. Put the .mobi files in the root of /Books.
I've got Calibre 0.8.69. I have enabled the beta drivers and am able to send books to the Fire HD via MTP.
I've confirmed the .mobi files are in the "Books" folder on the Fire HD. They are in subfolders with the authors name.
However, the books don't show up on the Fire HD anywhere. I've looked in books and docs and I have rebooted the device several times.
I have successfully sent the books to the Fire HD using the @kindle.com email address. Those show up in the doc section on the Fire HD. Interestingly these appear to show up on the HD in the root of the "Books" folder with names like B003SX0XY4_EBOK.prc and B004MME2OC_EBOK.prc.
I didn't have this kind of trouble with the origional Kindle Fire.
Am I the only one? Is this just how the Fire HD works?