Go to the B&N app store. Get the OpenExplorer app. It will allow you to see the files that have been downloaded. (Note, some of these files are probably because of apps.)
What NookGardener said.
Nook HD splits p the available space 50-50.
50% is available to you for your files you sideload.
50% is for things bought from B&N like apps and books and magazines.
If the stuff in the'Others' folder is NEW i.e. it wasn't there in the beginning, then the likely culprit are apps. I can't think of any thing other than apps and movies that would take up multiple GB.