XmlDocument xdoc=new XmlDocument();
xdoc.Load(Server.MapPath("books.xml"));
XmlNode xnode = xdoc.SelectSingleNode("/Books/Boook/Title=Understanding XML");
string str = xnode.InnerXml;
XmlDocument doc = new XmlDocument();
doc.LoadXml(str);
doc.Save(Server.MapPath("bookstest.xml"));