You designed a book,
You created a "Saved Book Collection" by clicking on the tab [Create Saved Book],
You now have a "Book Collection" in the Collections panel that contains the Book photos.
One L-mouse click on this Book Collection will open the photos in a Library Grid view of the collection- Do any of these thumbnails show the "Missing" icon?
When you resume editing the book do you Double-click on the Book Collection to open it in the Book Module?
Thanks for your help. I read the post on the right column about Missing Source File and learned that dragging and dropping on a Mac can be problematic. I went back and re-imported one of the photos and lightroom re-connected to the others there in the same folder. All seems to be working fine now.
I guess the lesson learned is- Never Move, Rename, or Delete photo files with Mac Finder (or Windows Explorer). Always use the Lightroom Folder Panel for these functions, or Lightroom will lose its database connection to the files.