<?php
  // Only show if $match is true
  $match = false;
 
  // Which node types
  $types = array('book', 'news', 'anothernodetype' );
 
  // Match current node type with array of types
  if (arg(0) == 'node' && is_numeric(arg(1))) {
    $nid = arg(1);
    $node = node_load(array('nid' => $nid));
    $type = $node->type; 
    $match |= in_array($type, $types);
  }
 
  return $match;
?>