Q: Will my photo book images print in full resolution if I use 'optimize mac storage'?

Not true. You can download full sized photo files from iCloud.com photo by photo.

Import them into your library again and make an "album" of just those so you know they are full size for making your book. After your book is sent off, you can remove them if you wish.

Depending on the size of book's pages, and the photo's size on that page, the smaller resolution files may not even be an issue.

Yes - the needed ones will be downloaded and used in adequate resolution (not necessary full resolution - I believe that Photos will do a minimum of a 180 DPI image although that is not totally clear)

Preview the book using this method -  Preview a print project in Photos or iPhoto to avoid issues - Apple Support  -  and Apple will staand behing any quaity issues you preview

LN