You will not be able to just download most of your books without further steps. There are a number of options for this discussed on this site, including the use of PC programs, an Android app and a greasemonkey script. The Digital Reader even wrote an article about it.
http://the-digital-reader.com/2014/0...es-wont-allow/
If you then want to read your books on anything other than a Nook or a Nook sanctioned application you will need to remove the DRM. You should first satisfy yourself that it is legal to do so in your jurisdiction. If so, search for "Apprentice Alf".