I'm using CC 2014, and this error appears when I attempt to export a book without any of the files open. However, if I open one of the files in the book, only that file will export. So what I'm saying is why can I only export the ebook if all of the files in the document are open?
Thanks for the help!
This happens when your style source document is missing from the book. There should be a red mark to indicate that this document is missing from your book. Please either change the style source or relink it to an existing document to create book.
For the issue, only opened document is generating ebook. Please make sure you are trying to export via book context menu, not using file menu --> export..
Let me know if you are still facing the error..