I find it unfortunate that visitors to my shelf see a blank green cover with no title or author information for books with no cover picture, so I've been experimenting with a customization to remedy this. It's still in an early stage, but if anyone else feels like giving it a try, here's what to do:
1. Edit the template HTML for your Shelf page.
2. Find the following:
<a onclick="{{ row.popup }}">
<img src="{{ row.picture|picture(100, 155, 'book') }}" title="{{ row.title }} - {{ row.author }}" alt="{{ row.title }} - {{ row.author }}" />
</a>
3. Edit it to read:
<a onclick="{{ row.popup }}">
<img src="{{ row.picture|picture(100, 155, 'book') }}" title="{{ row.title }} - {{ row.author }}" alt="{{ row.title }} - {{ row.author }}" />
{% if row.picture == '' and row.title != '' %}
<div style="position:relative;width:80px;height:0px;top:-130px;margin:auto;">
<span style=font-size:9px;font-weight:bold;>{{ row.title }}</span>
{% if row.author %}
<span style=font-size:8px;font-style:italic;><br />by<br />{{ row.author }}</span>
{% endif %}
</div>
{% endif %}
</a>
4. Save your changes.
Visitors to your shelf will now see title and author displayed on the previously blank green cover.
I am likely to continue tinkering with this and will post an update if I make improvements.