I have the code:

class HomeController extends ApplicationController
{
var $models = 'User, Addressbook, Contact';

...................
}

class ContactController extends HomeController
{
function menu()
{
//print_r($this);
$this->layout = 'contact';
if( !empty($this->params['id']) ){
$param = $this->params['id'];
$options = array('conditions' => array("addressbook_id = $param"));
$this->contacts = $this->Contact->find('all',$options); // <--------
//Fatal error: Call to a member function find()
//on a non-object in /var/www/rubrica3/app/
//controllers/contact_controller.php on line 24
}
}
}

So take a look.

if I call the page 'www.mysite.com/contact/menu/9' appears this error:
Fatal error: Call to a member function find() on a non-object in /var/www/rubrica3/app/controllers/contact_controller.php on line 24
because $this->Contact is strangely not istanced,
else if I call the page 'www.mysite.com/contact/menu/' $this->Contact exists...

is it a bug or I made some mistake?

Model names should be in lowercase, i.e.

var $models = 'user, addressbook, contact';