Sorry, once you delete a book from your Nook it's gone unless you have them backed up somewhere else. You should always archive your books, do not delete them unless you sure you never want them again.
What gheid45 wrote.
Delete = Deleted from Account forever. You have to rebuy them to read them again.
The right thing to do to move them only from device is
Nook Tablet+Color - Perss and hold icon and in menu that comes up choose 'Archive'.
Nook HD& HD+ - Press and hold icon and in menu that comes up choose 'Move to Cloud'.
You can also press and hold volume down button (or perhaps up) and icon to get larger menu with Archive.
Thats just a pain in the butt, I dont want to have to repurchase them as I just bought them recently.
I'm starting to regret buying a Nook.