def to_xml_string(str = '')
      return if empty?
      str << "<bookViews>"
      each { |view| view.to_xml_string(str) }
      str << '</bookViews>'
    end
def to_xml_string(str = '')
      return if empty?
      str << "<bookViews>"
      each { |view| view.to_xml_string(str) }
      str << '</bookViews>'
    end
creates the book views object
 Serialize to xml
 @param [String] str
 @return [String]