Решил

function theme_name_preprocess_custom_pager(&$vars) {

$nav = $vars['nav_array'];
$prev = 'prev';
if (!empty($nav['prev'])) {
$nodeObj = node_load($nav['prev']);
if ($nodeObj) {
$prev = node_page_title($nodeObj);
}
}

$next = 'next';
if (!empty($nav['next'])) {
$nodeObj = node_load($nav['next']);
if ($nodeObj) {
$next = node_page_title($nodeObj);
}
}