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

Applies to : Windows/MAC

Solution:

Launch QXP 2017 >> Open the project >> go to File menu > Save a Copy As/ Downsave menu . Select version as current version i.e. 2017 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.