You might want to try connecting the Nook to a PC and seeing if you can find and delete the books with a file manager.
You can also download a file manager app from GoPS, such as ES File Explorer. The Nook app store has Open Explorer which is also good. These give you access to the contents of your Nook and any uSD card you may have installed. Note that the internal Nook storage is identified as "/mnt sdcard" and the uSD card is identified as "/mnt ext_sdcard".