Transporter is a command-line tool that can be used to deliver content and download reports. Only those who have been approved and trained to use Transporter have access to it. Additional information about Transporter can be found in the Transporter User Guide.
No, you can only submit one book at a time using iTunes Producer.
You can enter metadata for multiple books in the Book File Import Template, available in the File menu in iTunes Producer. Doing this, however, still requires you to add the cover art and book file, and deliver the book package to Apple one at a time via iTunes Producer. Learn more in the Apple Books Publisher User Guide.
Alternatively, you can contact an Apple-approved aggregator to submit multiple books for you.
When you prepare a book to be submitted to Apple Books using iTunes Producer, you are creating a book package with this file extension. When you open this file, you will see the metadata (metadata.xml), cover art file, and book file if supplied.
This file is saved to your Playlist folder by default.