Applies to: QuarkXPress 2019 and  QuarkXPress 2018

Background: If a project is already added as chapter in a book, you cannot add it again as a chapter in another book. Even if you remove the chapter from the previous book, the chapter cannot be added to another book.

Solution: You need to open the project and go to File menu > Save a Copy As/ Downsave menu . Select version as current version i.e. 2018 and select the layout and save it as project with a new name. Now you will be able to add the newly saved project as chapter in a new book.