private static void writeGuideReference(GuideReference reference, XmlSerializer serializer)
     throws IOException {
   if (reference != null) {
     serializer.startTag(NAMESPACE_OPF, OPFElements.REFERENCE);
     serializer.attribute(PREFIX_EMPTY, OPFAttributes.TYPE, reference.getType());
     serializer.attribute(PREFIX_EMPTY, OPFAttributes.HREF, reference.getCompleteHref());
     if (StringUtil.isNotBlank(reference.getTitle())) {
       serializer.attribute(PREFIX_EMPTY, OPFAttributes.TITLE, reference.getTitle());
     }
     serializer.endTag(NAMESPACE_OPF, OPFElements.REFERENCE);
   }
 }
  /**
   * Write a resource as an item element.
   *
   * @param serializer the XML serialiser to write the item element to
   * @param book the book to write the resource for
   * @param resource the resource to write as an item element
   * @throws IOException if an I/O error occurs
   */
  private static void writeItem(
      final XmlSerializer serializer, final Book book, final Resource resource) throws IOException {
    if (!isValidResource(book, resource)) {
      return;
    }

    serializer.startTag(NAMESPACE_OPF, OPFElements.ITEM);
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.ID, resource.getId());
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.HREF, resource.getHref());
    serializer.attribute(PREFIX_EMPTY, OPFAttributes.MEDIA_TYPE, resource.getMediaType().getName());
    if (StringUtil.isNotBlank(resource.getProperties())) {
      serializer.attribute(PREFIX_EMPTY, OPFAttributes.PROPERTIES, resource.getProperties());
    }
    serializer.endTag(NAMESPACE_OPF, OPFElements.ITEM);
  }