Formatting an ebook boxed set requires attention to detail. When you create the bookmarks for each chapter (which will be used to generate the table of contents) they must have unique names or it will crash. Each book needs its own set which operates independently, wherever the reader happens to be in the ebook. This can get tricky, but worth it so the final result works properly and gives a professional impression. If you can’t figure it out, then hire someone who can.