# Here's the whole RenderToMarkdown class inlined into the RenderToHTML.book method:

  def self.book
    [
      title,
      css,
      BOOK.sections.map do |section|
        [
          "## Part #{section.number}. #{section.name}",
          section.chapters.map do |chapter|
            [
              "### #{chapter.number}. #{chapter.title}",
              RenderExternalFormats.erb(chapter.content),
            ]
          end,
        ]
      end.flatten.map do |markdown|
        RenderExternalFormats.markdown(markdown)
      end
    ].flatten.join("\n")
  end