Thank you all for your help guys!
@MartyTX: I don't think the exercise was futile, it might just as well have worked :-) I did the search again, but do not find any .original_epub files.
@BR: I ran Recuva. It has found a good deal of the lost books. But unfortunately most are tagged unrecoverable because they have been overwritten :-(
Most books appear to be in the recycle bin, but some are in an unknown location D:\?\ What does that mean?
For those with "a green dot" can the epub simply be "recovered" by right-click and then magically all is well? :-)
@theducks: I have tried your suggestion. When I drag an epub from the explorer window to calibre (either book detail panel on the right-hand side in the main calibre window or the book-format box at the top-right in edit metadata, a +copy appears but nothing actually happens?