(1) get the registration right for DRM
Register it with your AdobeID (the same ID you used on the Sony). (Library/Authorize Computer)
Connect the Nook.
Register it with your AdobeID. (Library/Authorize Device .. that will copy the ID already associated with the computer)
(2) copy the books
Connect the Sony and the Nook at the same time.
They will both show up as drives on the computer.
Use file manager to copy the books from one to the other.
They will probably be in the 'Digital Editions' directory on both devices.
If you have already been using ADE or Sony Reader to get the books onto the Sony, they may already be on the computer.
In that case, you can just drag them from wherever they are on the computer onto the Nook.
You may be able to use ADE to do the copy if you prefer.
If you use ADE2.0 you may get issues.
If so try the older but more reliable 1.7.2.
Version 1.7.2, it is a little difficult to find, available on Adobe site for Windows and for Mac.
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