public function addBook(Book $book)
 {
     $cart = $this->getCart();
     // Check if paper version available before adding to cart
     if ($book->getHasPaper()) {
         $key = $this->getItem($book, $cart);
         if ($key !== FALSE) {
             $cart = $this->increaseItem($key, $cart);
         } else {
             $cart = $this->setItem($book, $cart);
         }
         $this->setCart($cart);
     }
 }