XML Document Loading

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using MSXML2;

namespace ManipulateXML
{
    public partial class Form1 : Form
    {
        private DOMDocument40 doc;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void listBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            string search = listBox1.SelectedItem.ToString();
            IXMLDOMNode nd = doc.selectSingleNode("bookstore/book[@ISBN=' " + search + " ' ]");
            MessageBox.Show(nd.text);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            doc = new DOMDocument40();
            doc.load("books.xml");
            IXMLDOMNodeList nodes;
            nodes = doc.selectNodes("bookstore/book");
            IXMLDOMNode node = nodes.nextNode();

            while (node != null)
            {
                listBox1.Items.Add(node.attributes.getNamedItem("ISBN").text);
                node = nodes.nextNode();
            }
        }
    }
}

this is a pretty simple code to understand. Button click loads an XML document called books.xml and I try to get information about the ISBN of all book nodes.
I compile and it compiles but on button click nothing happens. When i ran debugger I saw that 'while (node != null ' is a false statement. Please help