A few I haven't seen anyone else mention:
- Narrator(s): for audiobooks.
- Universe: Star Trek, Forgotten Realms, etc.
- Game System: I have RPG sourcebooks in here, so this holds the system(s) they apply to -- Pathfinder, D&D 5E, d20 Modern, etc.
- See Also: path to where related/bonus materials are on the file system. Some books I've Kickstarted have come with wallpapers or other digital assets, and some of the RPG books from Humble Bundles have separate zip files with tiles, maps, etc.
- Queues: tag-like field with values for each family member's reading lists. Used to also have a Homeschool queue when I was assigning homework to my kids.
Others have mentioned these or similar:
- Source: where I obtained the book. E.g. "StoryBundle.Dark Crimes" or "Humble.Audiobooks.Doctor Who and Torchwood by Big Finish" or just "Amazon". My favorite value for this field is "Kickstarted," I have a few of those. Supporting authors is the best.
- Processed? Yes/No (as in, have I cleaned up metadata and verified the file)
- Multiple Rating, Date Read, and Review fields for myself and my family members
- Version: this is mainly because sometimes I have multiple versions and I use this field to disambiguate them. For something like "2nd edition" I include that in the title, but if it's the same underlying source book but there were multiple digital releases, or for things like alternate covers of CBZ comics, I put information here because the rest of the metadata looks like a duplicate.
I might steal
Subscribed,
Awards and
In Collection (for short stories). Y'all have good ideas.