Preview shows the book as it will be; Export… writes it and opens Share: Save to Files, Books, Mail, AirDrop. Print… (⌘P) sends the pages to the printer.
iPad Help
Exporting
Export Book… (the share icon at the top, or ⇧⌘E) puts the book’s documents together, in the order of the sidebar, as Word, a manuscript PDF for publishers, a book PDF, ePub, HTML or Markdown.