How to set multiple-author metadata for an item in iBooks?
All this metadata exists on the filesystem, so you could use the xattr command line to change pretty much anything, but I find the documentation and syntax to be very challenging to get and use.

I would start by trying xattred editor and perhaps the easier tools SearchKey / SearchKeyLight


https://eclecticlight.co/downloads/


The author has a very superb blog explaining why the app was made in the first time and how to use it as well if you don’t get the buttons or features. Think of all these metadata as a key value store. You just name the attribute (like modification date) and then set a value (today’s date or whatever). Same for page count and in your case, the author metadata string type.

Since you are reading the overall catalog, I would focus on changing the metadata in each book one by one (perhaps working on a throwaway ebook or duplicate copy of a book if you don’t have a good backup and restore procedure for the books you store in iCloud).