title_body: ePub 3: Finding the Navigation document
upvoted_answer: I'll post an answer which seems to be working for me.
  
  I found relevant documentation here: http://www.idpf.org/epub/30/spec/epub30-publications.html#sec-package-documents
  
  Using Ruby and Nokogiri, I decompressed the ePub file, read the package document as HTML, then used an XPath statement:
  
  unzipped_file   = Zip::File.open(epub_file)
  package_file    = unzipped_file.glob("*/package.opf").first
  package_as_html = Nokogiri::HTML(package_file.get_input_stream.read)
  package         = package_as_html.at_xpath("html/body/package")
  
  nav_file_name   = package.
                      at_xpath("manifest/item[@properties='nav']").
                      attribute("href").
                      text