I heard recently that you can find some books whose copyright has expired for free in the Ebook format. I am curious if this is true and if it is, if this will work on the Nook?
It should work in any format. Look for Project Gutenberg. Not only are they source for most of the books, but I think they will have instructions on how to get onto your reader. If they don't have the instructions on their site, then go to B&N's forums and you'll find plenty of help there. You should also be able to get a library card and borrow e-books from your local library, if you haven't tried that before.
Yes there are lots of free ebooks in the internet and I am sure they will work with Nook tablets because Nook tablets supports PDF and CHM formats of ebooks. I can't give you specific sites where you could get a lot of books but google will help you find lots of it.