I have see the same 'Progress Bar" in the past but cannot remember why!
Most likely answer is that Lightroom is searching through the full database (catalog) of photos before it builds the book.
I am without an answer, just Questions-
Do you have "All Photographs" selected in the Catalog panel under the Navigator?
Have you created a standard Collection of images to use in the book?
How many photos do you see on the Filmstrip at the base of the Book screen?
I note that the book is still in an "Unsaved" state, have you tried to [Create a Saved Book] and then always work from the "Book" Collection? (not the "Standard" collection)
WobertC thank you for the suggestions.Last night I was able to stop the 'Collecting Information' task from launching by saving the currently open test book using 'Create Saved Book.' I restarted LR and all was back to normal until I deleted the book from the Collections panel. This morning I created another small test book with 3 pages, used 'Create Saved Book,' switched to the Library module, and then deleted the book in the Collections panel. This allowed me to remove the test book without causing the 'Collecting Information' task to launch again. Thanks for your help!
If interested this is the issue I was working on when this problem started. It may or may not be related: