How Can I Preview A Book Spine?

I am confused as to how I can preview what the spine look like on my book made with Photos.

In Aperture, you had to go to the buy book option and then generate the PDF. Only on that document would you see the spine text on the single page images for the dust jacket and hard cover on the super wide preview image of those pages

The preview that Photos generates when select "Save Book As PDF" doesn't have this. The front cover, back cover, and inner dust jacket images are their own pages. I am not getting the preview image that includes the spine.

I am aware that you have to add text to the cover for it to show up on the spine. I've doing a Large Hardcover book as well, so it should have a spine to it. I am replaced the placeholder text too.

Any ideas what I am doing wrong?