Is the Nook a dedicated eReader or a tablet device?
For a dedicated eReader deivce, which it is plugged into the computer with USB, it should show up as a device in a left hand panel of ADE.
Use menu/help/Authorize Device, and it should then authorize the device with the same ID as the computer.
For a tablet, you will probably need to do the authorization using the application you are running on the device.
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In either case, it is important that ADE itself is authorized with an explicit Adobe ID.
You have a choice when you first start ADE of authorizing with such an ID, or running without ID.
If you run without ID lots of horrible things happen, including that you can't read books dowloaded to ADE anywhere else.
If you are in that situation:
- remove DRM from the books you have already downloaded using epubee or similar (****)
- get yourself a free AdobeID from adobe.com ~ try to log in and it will lead you to a new id screen
- authorize ADE with the Adobe ID (menu/help/authoize computer)
- authorize the device (as above)
- copy the DRM free version of the old books
- any new books should work properly
(****) I do NOT suggest the use of DRM stripping in general to break the publisher's rights and avoid the intent of the DRM. However, where the extremely bad implementation of the Adobe ADEPT DRM infrastructure leads to users being cut off from proper use of their books I think its use is completely justified.
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You may have hit a combination of device and publisher that does not work properly with ADE2.0. This is quite common, particularly with librry books.
There are lots of bugs in ADE2.0. Try replacing ADE2.0 with the older but more reliable v1.7.2.
(You can have them both installed at once if you like.)
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
http://helpx.adobe.com/digital-editions/kb/cant-install-digital-editio ns.html
The forum software is sometimes corrupting the link above. There shouldn't be a blank in 'editio ns.html'. The following redirects to the same page: http://tinyurl.com/diged172