Javascript JSON.stringify() not working

OK I have spend alot of time trying to figure this out. I am reading a cookie in PHP and and storing it inside an XML file. The cookie is of type array of objects. I then want to set the cookie stored in the xml file with java script. You might ask why I am doing this, I am storing some user information inside the xml file, and i want to set the cookie when the user logs back in.

I am having trouble setting the cookie with Javascript. The JSON.stringify() doesnt seem to be working here. When I do document.cookie in the browser console the cookie is of type object.

Here is my PHP code:

if(isset($_GET['save'])){
   $books = $_COOKIE['myBooks'];
   $username = $_SESSION['user'];
   $users = simplexml_load_file("users.xml");

   $user = $users->addChild('user');
   $user->addChild('username', $username);
   $user->addChild('books', $books);

   $users->saveXML("users.xml");
}

This all seems to be working fine. Here is my javascript:

function getPapers(){
    delete Array.prototype.toJSON;

    loadXML("users.xml", function(xml){

        $(xml).find("user").each(function(){
            var books = $(this).find("books");
            books = decodeURIComponent(books);
            books = JSON.stringify(books);  //Not working
            alert(typeof(books)); //returns object
            document.cookie = "myBooks=" + books;
        })
    });

}

This is the content of books: "[{"bookName":"The Whale","bookCode":"THW12","copies":"1"}]"

Am i doing something wrong? I need the cookie to be stored as a JSON string.