I’m afraid I don’t understand your question, so forgive me if I’m wrong.
If you want to add the authors of the books, I believe the best thing to do is to use a custom post type for the authors and then you can use a post object field type to set the author. By using a custom post type, you can specify more fields and custom query in the future.
Also, if a book has more than one authors, you can set the “Select multiple values?” option on the post object field or use a repeater fields for the authors.
I hope this helps 🙂
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