The book is not lost. This situation is caused by your choice for download
when you're getting an ebook from Gutenberg. You should choose 'SAVE', not
'OPEN'. If you choose OPEN, ADE thinks you want to read the book. When it
appears in Reading View, it's physically on the computer, but has not been
put into the Library.
You can add the book to the Library by opening ADE in Library mode. Next to
the word 'Library' is a small arrow. Put your cursor on that arrow, and a
drop-down menu will appear with the choice 'Add Item To Library'. Click on
this entry, and a screen will appear allowing you to find the book you want
to add. When you've found it (usually in the Digital.Editions folder),
click on its name, then click OK and the item will be added. You can check
on it by refreshing the Library screen using the F5 key on your PC.
Next, you said the Nook doesn't show up in ADE. That could be from a couple
of causes - both dealing with the process to get ADE to recognize the Nook.
The first step is registering the Nook on the B&N website. When you do
this, use your Adobe ID and password - it will make things much easier
later. The next step is to attach the Nook to your PC. Plug in the Nook to
a USB port and make sure that Windows sees it - usually as a hard drive.
Once it does, then start ADE. After ADE opens, you should see the Nook in
the bookshelf section (left panel) of the Library view. Locate a book you
want to load to it. Then, you can drag and drop the book to your Nook.
While this sounds easy, it's the order of steps that is needed - if you do
it differently, it might not work.
Once you've done all the transfers you want, shut down ADE but leave the
Nook connected. It will do some synchronization, adding the new material to
its own internal indexes. When it tells you it's done, then eject it from
your PC logically either by choosing 'Eject' from the My Computer menu or
'Safely Remove Hardware' from the Start bar's icon in the lower right hand
corner of your display.
This is a pretty high level description and if you have more questions, just
post them. Hope this helps!