What do all these fields in iTunes Producer mean?

Check out the Apple Books Publisher User Guide for more information on book metadata.

Do I need an ISBN? Can I use an old one or the print version?

ISBNs are optional, but helpful for sales and charting purposes. If you choose to use an ISBN, you must provide it the first time you submit the book to Apple. All ISBNs must be in the 13-digit format and be specific to the digital version of the book. Learn more about purchasing an ISBN in the United States and other areas.