Q: Copy a Fotobookprojekt to Finder Programm

Create a PDF preview of the finished book and save this to a folder on your Desktop.

This document explains how to create the preview:

Preview a print project in Photos, iPhoto, or Aperture to avoid issues - Apple Support

You cannot export the project itself.  If you want to save the project as as a project. duplicate the iPhoto Library and delete everything from the copy but the book and the photos used in the book.