Stylize book index with images
You actually don't need a module to do this

The previous/next links can be directly modified by 'just' overriding the file modules/books/book-navigation.tpl 

&lt;?php if ($has_links): ?&gt;
    &lt;div class="page-links clearfix"&gt;
      &lt;?php if ($prev_url): ?&gt;
        &lt;a href="&lt;?php print $prev_url; ?&gt;" class="page-previous" title="&lt;?php print t('Go to previous page'); ?&gt;"&gt;&lt;?php print t('‹ ') . $prev_title; ?&gt;&lt;/a&gt;
      &lt;?php endif; ?&gt;
      &lt;?php if ($parent_url): ?&gt;
        &lt;a href="&lt;?php print $parent_url; ?&gt;" class="page-up" title="&lt;?php print t('Go to parent page'); ?&gt;"&gt;&lt;?php print t('up'); ?&gt;&lt;/a&gt;
      &lt;?php endif; ?&gt;
      &lt;?php if ($next_url): ?&gt;
        &lt;a href="&lt;?php print $next_url; ?&gt;" class="page-next" title="&lt;?php print t('Go to next page'); ?&gt;"&gt;&lt;?php print $next_title . t(' ›'); ?&gt;&lt;/a&gt;
      &lt;?php endif; ?&gt;
    &lt;/div&gt;
&lt;?php endif; ?&gt;


this is also described a bit more detailled in this answer to a very similar question:

Next/Previous button for nodes within a book

The book tree is quite a bit more complex. 
A good start would probably be to look at the functions template_preprocess_book_navigation and book_children and menu_tree_output and then override the parts you want to 'style' (theme) in your template.php – but that can be tough.

I ran into a similar problem a while ago – and not using the book module was the solution for me. Instead I dived into Entity reference and found it a way more flexible approach than using and theming the book module.