Unfortunately the formats for a few books got crossed, so a merge would delete some books.. It's the export/import method then.
I have so many books that date back 15+ years and backups of backups of backups of backups, so tons of duplicates and stuff that no automated system could ever figure out. When I imported them, I set it to keep all files and it appears to have created records and filled in differnt formats in the first available slot. At least I'm finally starting to get them organized and cleaned up

Approx two months down, 1000 books absolutely verified (a custom field to track my progress), 4000+ duplicates deleted and about 9000 left to go lol.
And yes, I've become very familiar with the merge and bulk metadata functions.