Converting books into the EPUB3 format is a job for professionals who know HTML, but above all it demands an orderly, clean workflow that follows the proper standards.
Here in Israel there are a number of book editors, some more professional than others. For those who are not yet familiar with it (and there are some), here is a link to a tool called epubcheck - just drag the finished file onto the interface. If it comes back green and with no warnings, the file is valid as an EPUB3 book. Good luck!
http://www.pagina-online.de/produkte/epub-checker/
