Hi Nicky and welcome to Mobileread! As you will see there are quite a few areas around here where you can find out ways of buying books from different ebook stores. You're not stuck with B&N just because you have a Nook. There are a few threads that give lists on other ebook buying websites, just do a quick forum search
Here is the link to the Reading Recommendations forum if you need some good recommendations
https://www.mobileread.com/forums/forumdisplay.php?f=24