I have about 700 books in my library. Thanks for the Modify Plugin tip. I installed it and have experimented. When I put a book that I'd used the plugin on, the cover appeared in my Nook. So it solved my problem on no covers showing in my Nook.
And thanks for the shortcut to deleting items from the library. I've copied my library and I will work on the copy to clean up the double epub book issue. I keep my library in Dropbox, so I don't want it to get too large.
Appreciate all the help.