What if that book was actually already conceived by the devs? Pagies doing something similar to Jiggies in completing a puzzle makes sense.
Imagine you can hop into an incomplete book. What I mean is, once you complete the first chapter of a book, you can play that level. Then you can come back with more pagies from other worlds to open up the second or third chapter - expanding the world.
Would be a great surprise if there's no indication, or only a very subtle one, that there are still pagies to put into the book.