It depends upon how much you have backed up.
If you backed up the entire contents of the (My) "Digital Editions" folder (including manifest.xml), then restoring that folder, and then installing ADE should get you back to your previous state.
If you just backed up the .pdf and .epub files, then you will need to install ADE, and then import those files back into ADE (Library->Add Item To Library - you can multiselect btw, for more than a couple hundred books, be patient it make take 10-20 secs to get everything in).
To install ADE, go to
http://www.adobe.com/products/digitaleditions and click on the big button that says Install
If you have any problems let us know.