In the collections panel right-click (control-click on Mac) on your saved book and choose Duplicate Book. That will automatically create a second saved book with the word copy added to the name. Add photos to this copy book, add pages and drag the new photos to the new pages.
Once you are happy that everything is retained you can either delete or rename the original by right-clicking.
In practice I've found it best not to delete, where the collections have shared photos, at least until the book is printed.