No direct downloading as far as I know. As long as the books are in epub format with no DRM all you gotta do is download the epub files to your pc. Connect the Nook to the PC with the USB cable, a window will pop up. Browse to the My Documents folder and copy the epubs to there.
This video shows the process too:
http://www.youtube.com/watch?v=JHNEGlayEIQ
Also, some of the old free ebooks are available from the barnes & noble site for free. If you "purchase" one of those free ebooks then it will auto download to your nook library.
Also, I haven't used feedbooks to get them, but I know you can download the epub files from google books here:
http://books.google.com/books?hl=en&...okshelf_slider