As soon as you save the files in the book folder, Dropbox will pick them up (assuming it is running) and the little Dropbox icon will look like a spinning thing. Once it's back to the green checkmark, Dropbox has the files. (Or, just wait about 10-20 seconds.) After this is true, clicking the publish button will trigger book generation with the latest files, since we always get the latest files from Dropbox.