What is the best way to generate node(book) to ePub?
I know this answer is dead simple, but have you tried EPUB Export module?


  This project builds content objects from drupal and bundles them into an
  EPUB 3.0 document. Currently it will export a rendered entity seperate of all
  of the page markup and gather all the referenced resources within the content
  and bundle those into one EPUB document.


I tested it on a clean install and it appears to work as advertised.