Q: my Photos book project disappeared

First repair the library according to the instructions in this Apple document: Repair Your Library.

Note: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).

Next, if necessary, restore your latest backup copy of the library made just previous to this occurring and see if the book is in the library and workable.  You can restore the library to another location so you don't overwrite your current library. Or rename your current library so the restore doesn't overwrite it.

Do you have a backup copy of your library created just prior to this occurring?

Try restoring it to a different location that your current working library and see if the book is still there fully or in part.