One possibility ... Sometimes ADE gets its registration/activation confused and in a semi-authorized state. Unfortunately, it often then gives misleading error messages about what is wrong. A common incorrect message informs you that the ID is already in use on another computer and cannot be reused. This can often be resolved by completely removing any authorization using ctrl-shift-D to the Library screen on ADE (cmd-shift-D if on Mac). Restart ADE, and then reauthorize with your (old) Adobe ID. In extreme cases on the mac, the following extra step has helped some people. Navigate to /Users//Library/Application Support/Adobe/Digital Editions and drag the activation.dat file to the trash. If you are using 10.7, see Access hidden user library files | Mac OS 10.7 Lion. http://forums.adobe.com/thread/1265248?tstart=0
~~~
To save all your books, goto to your ADE directory
(probably in something like Documents\My Digital Editions or C:\Users\<userid>\Documents\My Digital Editions)
and copy all the .epub files (and .pdf files if you are unlucky enough to have any) to a backup device.
If you copy the entire directory it will remember things like your last reading place in books, and notes you have made).