New second (optional) parameter in function nm_custom_display_recent (and nm_custom_list_recent) that lets you filter by tag.
Example:
<?php nm_custom_display_recent('
<div class="my_recent_post">
<h4><a href="{{ post_link }}">{{ post_title }}</a></h4>
<span class="my_date">{{ post_date }}</span>
<span class="my_excerpt">{{ post_excerpt }}</span>
</div>
', 'books'); ?>
will display the latest posts with tag books (using the sample layout given as first parameter).