You can use Adobe Acrobat Pro or other PDF stiching tools to combine multiple PDF's into a single file.  You would want to do this if your publisher or designer gave you multiple files for your book.  If possible, try to get a single file from them, as they can probably combine the pages for you very quickly.  

If you still have trouble and don't have resources to figure this out, please contact us and we can try and help you with it.