Good for your mother! How exciting! My mom has a Sony ereader and really likes it. Since your profile says you have a Kindle, then you should be able to provide decent tech support just because many things about ereading go across brands. I would download the user guide and maybe watch some of the videos at the B&N website to become familiar with the Nook. You could also visit a B&N or Best Buy and play around with the demo unit.
http://www.barnesandnoble.com/nook/support/index.asp
Some libraries have tutorials on their websites which explain how to download ebooks and transfer them to your device. I would visit her library website and see if there is something like that to read about how it works.
Of course there is always MobileRead to help answer questions that you can't figure out on your own.
