For books you have purchased from Barnes & Noble you should be using the archive function instead of delete. The archive function removes the publication from your hardware reader while allowing you to download it from the B&N servers at some point in the future as long as B&N retains a license from the publisher to distribute the book. Books can be deleted from your online library (and your hardware reader's list of archived items) by accessing your BN library with a web browser (scripts and cookies will probably have to be permitted). Once a purchased eBook is deleted from your online library you will have to purchase it again (with any associated costs) to download it again.
Since many desktop and laptop computers have much more storage than a typical eBook reader it may be preferable to leave your purchased and free eBook files on the computer's permanent storage (usually a hard drive). Of course there would be no point to retaining any eBooks lent/rented to you by a library or other source after the authorized reading period.
Sideloaded eBooks on you hardware reader can be removed using your computer's file management utility (Windows Explorer for computers using Microsoft's Windows Operating System) while your hardware reader is connected to one of the computer's USB ports. Make sure you properly "eject" the Nook's virtual drive before disconnecting it from the computer (for Windows Explorer right click the drive to reach the "eject" option on the context menu)
