gitbook/templates/includes/book/summary.html

<div class="book-summary">
    <ul class="summary">
    <li>
        <a href="{{ basePath }}/README.html">Introduction</a>
    </li>
    <li>
        <a href="https://github.com/{{ owner }}" target="blank">About the author</a>
    </li>
    <li class="divider"></li>
    {% for item in summary.chapters %}
        <li>
            <a href="{{ basePath }}/{{ item.chapter.path }}">{{ item.chapter.title }}</a>
            {% if item.articles.length > 0 %}
            <ul class="articles">
            {% for article in item.articles %}
                <li>
                    <a href="{{ basePath }}/{{ article.path }}">{{ article.title }}</a>
                </li>
            {% endfor %}
            </ul>
            {% endif %}
        </li>
    {% endfor %}
    </ul>
</div>