How to add a 'read more' theme link to a custom view mode?
I took a look here

https://api.drupal.org/api/drupal/modules%21node%21node.module/function/node_build_content/7

and here

https://drupal.org/node/1844292

and came up with this

function YOURMODULE_node_view_alter(&amp;$build) {
    if ($build['#view_mode'] == 'YOURVIEWMODE') {
        $node=$build['#node'];
        $links=array();     
        // Read more for teas ext
        $node_title_stripped = strip_tags($node-&gt;title);
        $links['node-readmore'] = array(
            'title' =&gt; t('Read more&lt;span class="element-invisible"&gt; about @title&lt;/span&gt;', array('@title' =&gt; $node_title_stripped)),
            'href' =&gt; 'node/' . $node-&gt;nid,
            'html' =&gt; TRUE,
            'attributes' =&gt; array('rel' =&gt; 'tag', 'title' =&gt; $node_title_stripped)
        );
        // Assign links
        $build['links']['node']['#links']=$links;
    }
}