If you download a book before authorizing ADE, then, the book is registered to a temporary user.
If you haven't authorized ADE yet, don't do it.
Google for inept.
This thread is you friend, too :
https://www.mobileread.com/forums/showthread.php?t=39423
Then, inept should, hopefully, be able to get the temporary key, and get rid of the drm. But, if you have no copy of the file you originally downloaded, that won't help much.
If you have authorized ADE afterwards, basically, you're screwed. Try mailing both the shop and adobe's support, maybe one or the other can help.
Good luck with that.