XML LINQ C#

using System;
using System.Linq;
using System.Collections;
using System.Collections.Generic;
using System.Xml;
using System.Xml.Linq;
public class MainClass {
    public static void Main() {
        XElement firstParticipant;
        XDocument xDocument  = new XDocument(new XDeclaration("1.0", "UTF-8", "yes"),
     new XDocumentType("Books", null, "Books.dtd", null),
     new XProcessingInstruction("Book", "out-of-print"),
     new XElement("Books", firstParticipant =
     new XElement("Book",
     new XAttribute("type", "Author"),
     new XElement("FirstName", "J"),
     new XElement("LastName", "R")),
     new XElement("Book",
     new XAttribute("type", "Author"),
     new XElement("FirstName", "E"),
     new XElement("LastName", "B"))));
        foreach (XNode node in firstParticipant.Nodes()) {
            Console.WriteLine(node);
        }
    }
}