guttenberg downloads will always be OK, they don't have ay DRM.
(You will probably find it better to use the free Calibre for those, it is far superior to ADE in all details except lack of support for DRM)
Does this happen for all library books?
Do you have an eReader plugged in ~~ there are certain bugs that prevent ADE2.0 working with some ereaders in combination with library books (including but not onky Sony PRS-350).
There are lots of other bugs in ADE2 as well, try 1.7.2
There are lots of bugs in ADE2.0 (and 2.0.1). 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.
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
Some people have found ADE trying to upgrade automatically.
It appears (not 100% sure) that if you install ADE2.0 as a new install (not as an upgrade) that your 1.7.2 will continue to run.
Probably best to say no if 2.0.x installation asks if you want to migrate your library.