private void AddSpineElement(string id)
{
XElement Spine = GetXmlElement("spine");
if (Spine == null)
{
Spine = new XElement(ns + "spine", new XAttribute("toc", GetNCXid()));
}
Spine.Add(
new XElement(ns + "itemref",
new XAttribute("idref", id)));
ReplaceSpine(Spine);
}
