"One or more of the files in your FrameMaker book is not a FrameMaker document. This file will be ignored while FrameMaker updates the book."
Nonsense, of course, but I notice the file in question (not always the same file) temporarily loses its .fm icon in the book panel.
My guess would be, network problems slowing the writing of updates; any comments before I mildly mention this to our IT Support?