When you add content (such as HTML widgets) to your iBooks Author books, make sure none of the files contained in the bundle include control characters (such as carriage returns) in the filenames. If such files are included, you won't be able to publish your book to the iBooks Store.
Note: Folders with custom icons sometimes include invisible files whose filenames contain control characters. For this reason, avoid using custom folder icons.
If, for example, you have downloaded an HTML widget from the web, and have modified it to use within your book, in addition to removing control characters from any filenames within the bundle, you will need to make sure that references to those files within the bundle have been corrected so that your HTML widget will function properly.
While HTML widgets are most likely to contain files that include control characters in the names, this may occur with files included in Collada packages or Keynote presentations as well.
Learn more about creating HTML widgets for use with iBooks Author.