Usually, a Drupal site will come with Navigation block. However, it is not enough for complex categories just like this site. So, I modified book-like navigation block to show only related contents.

// configuration
$myvocab = 3;
$mylimit = 10;
$includecurrent = false;
// code
$mycategory = array();
switch (arg(0)) {
  case 'node':
    $current = arg(1);
    $cats = taxonomy_node_get_terms_by_vocabulary($current, $myvocab);
    foreach ($cats as $cat) {
      $mycategory[] = $cat->tid;
    }
    break;
  case 'taxonomy':
    $current = 0;
    $mycategory[] = arg(2);
    break;
  default:
    $current = 0;
    $mycategory[] = 0;
    break;
}
$terms = taxonomy_get_tree($myvocab);
 
$operator = "or";
$result = taxonomy_select_nodes($mycategory, $operator);
$count = 0;
$items = array();
while ($obj = db_fetch_object($result)) {
  $node = node_load(array('nid' => $obj->nid));
  if ($node->nid != $current || $includecurrent) {
    $items[] = l($node->title, "node/". $node->nid);
    $count++;
    if ($count >= $mylimit) {
      break;
    }
  }
}
if (empty($items)) {
  return '';
}
return theme('item_list', $items);