Shortcode Ausgabe Fehlermeldung bei DB Abfrage

function listbooks_handler() {
   //run function that actually does the work of the plugin
   $bookoutput = listbooks_function();
   //send back text to replace shortcode in post
   return $bookoutput;
}
function listbooks_function() {
 // Datensätze holen
 
 $table_name = $wpdb->prefix . "mybooks";
 print "<h1>Wanderf&uuml;hrer &Uuml;bersicht ...</h1>";
   // Daten ausgeben
   $listbookdata = $wpdb->get_results( 
   "
   SELECT book_id, book_title
   FROM $table_name
   ORDER BY book_id ASC
   "
   );
   
 print "<ul id=\"Navigation\">";
  
   foreach ( $listbookdata as $listbooks ) 
   {
   $book_id = $listbooks->book_id;
  $book_title = $listbooks->book_title;
  
  print"<li><a href=\"book.htm\">{$book_title}</a></li>";
  
 }
 
 print "</ul>";
}
?>