ADE vs A9 battle of the book download

I am trying to download books from the library so I can transfer them to my new nook.   Every time I try to download to Adobe Digital Edition, Adobe 9 opens and says file corrupted cannot open. I have tried to make ADE the default but don't know how, it isn't in properties.  Anyone have any advise. I am struggling with this new nook because it won't download to Overdrive Media either.

Mo