The eBook files were fine. The metadata for the eBooks that was kept in the master metadata.db file was corrupt. The metadata for the individual eBooks, kept as part of each eBook directory, was not corrupt, so Calibre was able to rebuild the master metadata.db file from the eBooks themselves. Although I deleted the seven books from Calibre before rebuilding the database, that actually was not necessary. I was able to add all seven eBooks back into Calibre after the rebuild.

The corruption in all seven books was the same: even though the eBooks were physically present in the library with clean metadata, the master metadata.db file said there was no valid eBook format available for that particular eBook.

tanstaafl.