I don’t have a B&N Nook, but I’m told that these steps will allow you to sideload a .epub file into a Nook e-reader:
- Connect Adroid phone (or Nook e-reader) to PC.
 - Open “NOOK” folder
 - Open “My Documents”
 - Drag the .epub file to the “My Documents” folder
 - Disconnect the device
 - Open “Nook” application on the smart phone or power on the Nook e-reader
 - Filter by “My files”
 - Your sideloaded book should appear