warning sign in book panel

When a book file is open and you modify a file in that book the book file will note the change (number of pages, starting page number, styles, swatches, etc.). This is needed to make sure the book file can keep everything up to date, for example if the second document is supposed to continue page numbering then the book file needs to know how many pages are in the first file and what page number it ends with. If you modify the InDesign document while the book file is not open InDesign will not know what has changed and how that affects the rest of the book, even if nothing significant changed. Open the book file and then double-click on any documents with warning flags. This will open them and update the Book file.