ADE often gets its authorization confused.
First return the library book ~ it is probably now authorized with the mixed-up id, and you might not be able to return it once your proper ID is registered.
Clear any old authorization using ctrl-shift-D to ADE (cmd-shift-D if on Mac).
Restart ADE.
Try again to authorize ADE with your Adobe ID.
Then you should be able to set the ID into the Nook as well; and borrow the book back again.
Good luck ~ what a rubbish implementation of DRM Adobe has produced....
