Add Photos from Shared Album to a 'Project' in Photos?

Hi,

So I went to Ireland with the family and we all took pictures and created a shared album. I would like to create a hardcover book from the shared album and am finding that I can only add photos to the 'Project' for the book that I actually took on my phone. Is there a reason I cannot add photos from the Shared album that I didn't take to the 'Project' in Photos? Any help would be greatly appreciated. It is for a Fathers Day gift 🙂

Erik