Two separate cases, DRM e-books and non-DRM e-books.
For non-DRM e-books I suggest you use the free Calibre software; though ADE (Adobe Digital Editions) should work as well as below.
For DRM e-books you need ADE,
- Make sure your ADE is registered with an Adobe ID.
- If not, use ctrl-shift-D to deregister the 'anonymous' registration it might be using, restart ADE, and register with an Adobe ID.
You will be sent to the Adobe web site to get the ID if you do not already have one. - Plug in the Nook. It should show up on the left side of ADE.
- When you start ADE will probably come up with a request to register the Nook.
If so, follow it. If not, there should be a dropdown menu item for 'Register Device' (or maybe 'Authorise device') - You can now drag books from the ADE library onto where the device shows up on the left panel of ADE;
- and read them on the Nook once unplugged.
Unfortunately there are various things that might go wrong.
Try the above, and if it doesn't work report back exactly what issues you get.