This has found the file I searched for at this location:
/Users/richardtanswell/Pictures/Photos\ Library.photoslibrary/Masters/2015/11/22/Australia\ Oct\ Nov\ 2014
That is good. The photo is still in your Photos Library in the Masters folder. The problem is, that Photos cannot find the files. You could try, if repairing the library will fix this. It helped in iPhoto in similar cases, but in Photos it rarely helps.
Since it is only one album, I would copy the photos from the Masters folder to a folder on your Desktop and then reimport them. Test to reimport first with a single photo and see, if you can use the reimported version for your Book.
To open the containing folder for the photo you can use the search result in Find Any File. When you ctrl-click the found photo in the list, you can select to reveal it in the Finder.
When you import the photo again to Photos, you may see a warning about a duplicate being added. Confirm, that you want the photo to be added anyway.
Since the photos have been included in the library, stored in the Masters folder, and are not referenced outside the library, the command "File > Show referenced File in Finder" does not work, because Photos handles them as managed files. And so the command to "Consolidate" is also not available.
