Welcome to the forum.
I strongly recommend you use Calibre to manage your library. I added a custom column to mark the books I have read. Or, if you don't rate your books, you can use the existing star rating column to indicate whether the book is read, soon to be read, etc.
I don't like to sort through a lot of books on my NST to decide what to read next, so I seldom have more than 30 books on it at any given time. I delete each book from the reader when I'm finished - using Calibre - and then side load a new book. It works for me.
Enjoy your Nook,