Thanks everyone for the tip about linking my PFD format ebooks to the external reader. That works out very well.
The missing ebooks were in a library that was moved. The .opf files are still in the library, just no ebooks or covers. "Check Library" does not report the ebooks as missing, and it does report extra authors. Looking at the Windows Explorer revealed folders that do not show up in Calibre's lists.
I can easily get these ebooks replaced from the publisher. What method would be best for putting the ebooks back into their folders? Should I copy the ebooks to the file folders (Windows Explorer) and then run some sort of maintenance? Or better to "remove" the existing reference to the ebook using Calibre, and start fresh with new "add books"?
Is there a detailed user guide for the maintenance functions?
Thank you so much for your help.