Moving this discussion to the Adobe Digital Editions forum.
Please open the ACSM file in Adobe Digital Editions. Adobe Digital Editions will then download the PDF/e-book/ePub and make it available within Adobe Digital Editions.
As Jeff says, public library books normally initially download as a .acsm file, and ADE automatically downloads the real .pdf or .epub file when opening the .acsm.
When things are working properly this goes smoothly enough that you might not have noticed the .acsm files.
(Look in you downloads folder and yout might very well see lots of old .acsm files).
There have been a few bugs in ADE2.0. If you don't get a working solution with v2.0, try uninstalling and reinstalling 1.7.2.
(I think the change from v2.0 to v1.7.2 is the upgrade, but many people must be using V2.0 very happily.)
Version 1.7.2 is a little difficult to find, available on Adobe site for Windows and for Mac.
THANK your response as your solution and idea has assisted me in opening the files and loading on my Nook. Happy New Year to you and yours.