{% for book in object.book_set.all %}
<li>
<a href="{{ book.get_absolute_url }}" title="{{ book }}">
- <img src="{{ book.cover.url }}?{{ object.etag }}" alt="{{ book }}">
+ {% if book.cover %}
+ <img src="{{ book.cover.url }}?{{ object.etag }}" alt="{{ book }}">
+ {% else %}
+ {{ book }}
+ {% endif %}
</a>
</li>
{% endfor %}