OK, I think I'm starting to understand how this works now. My understanding is:
1. Certain types of files can be accessed directly from the SD card. Such files include videos, music, and photographs.
2. For other types of files, it's necessary to copy them to the appropriate app directory in the internal storage in order for them to be seen. This includes Kindle books. In that respect, it's the same as "Kindle for PC", which can only read books which are in the "My Kindle Content" folder.
Can anyone confirm that this is correct?