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);
}
}