'Documents' and 'Books' in the Kindle Fire are really the same thing if you think about it -- converted "mobi" (something more proprietary than that, i forget) files that the Kindle App can read. If you were to ever browse to your sdcard you would find scrambled encrypted pdf files in your book folder as well as your documents folder. It's completely random.
Anyway, I've had mixed success sending mobi files from Calibre to main memory (and having it sorted nto books) but lots of success dumping .mobi files into the 'Books' folder on your usb mount sdcard.
Either way -- its whispernet that delivers it and be it books or documents (especially after 6.3 when it comes to documents) they will end up in your cloud and only removable via amazon.com/myk
I'm a sole kindle fire owner too myself so PM me if you have anymore questions.