{% for index, authors in books_nav.items %}
  <ul>
    <li><a class="book-list-index" href="#">{{ index|upper }}</a></li>
    <li>
      <ul class="book-list-show-index">
      {% for author in authors %}
        <li><a href="#{{ author.slug }}">{{ author }}</a></li>
      {% endfor %}
      </ul>
    </li>
  </ul>
{% endfor %}