https://gist.github.com/bleathem/5360926 rendered with asciidoctor

book-multi.asciidoctor.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN" "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">

<book lang="en">
  <bookinfo>
    <title>Multi-Part Book Title Goes Here</title>
    <date>2013-04-10</date>
  </bookinfo>
  <chapter id="_the_first_part_of_the_book">
    <title>The First Part of the Book</title>
    <chapter id="_the_first_chapter">
      <title>The First Chapter</title>
      <simpara>Chapters can be grouped by preceeding them with a level 0 Book Part title.</simpara>
      <section id="_chapter_sub_section">
        <title>Chapter Sub-section</title>
        <simpara>Sub-section at level 3.</simpara>
        <section id="_chapter_sub_section_2">
          <title>Chapter Sub-section</title>
          <simpara>Sub-section at level 4.</simpara>
        </section>
      </section>
    </chapter>
    <chapter id="_the_second_chapter">
      <title>The Second Chapter</title>
      <simpara>An example link to anchor at start of the <link linkend="X1">first sub-section</link>.</simpara>
    </chapter>
  </chapter>
  <chapter id="_the_second_part_of_the_book">
    <title>The Second Part of the Book</title>
    <chapter id="_the_first_chapter_of_the_second_part">
      <title>The First Chapter of the Second Part</title>
      <simpara>Chapters grouped into book parts are at level 1 and can contain sub-sections.</simpara>
    </chapter>
  </chapter>
</book>