Originally Posted by dwanthny
Those other metadata files are all opf files and yes these files could be used to rebuild your metadata.db (calibre database) by using the Restore database menu option under Library maintenance (see attached).
The reconstructed metadata.db file should have all the original book related info. It won't have an non-book related info. My primary concern about non-book related info that will be missing is custom recipes. I have many of them.
Custom columns are also stored there and won't get recovered. I run a separate backup solely on the metadata.db file, in conjunction with a full backup of all books and the metadata.db file.
Personally, I'd never use the Restore database menu option as it wouldn't give me what I want most - recipes. I'd simply use my best backed up metadata.db (with my recipes) and then run an error analysis on the db to find books that are in the folders, but not in the db.