生成NCX文件内容
        :return:
        :rtype:

def gen_ncx(self, book_idx):
        """
        生成NCX文件内容
        :return:
        :rtype:
        """
        start, end = self._start_end_of_index(book_idx)
        ncx_base = """<?xml version="1.0"?>
<!DOCTYPE ncx PUBLIC "-//NISO//DTD ncx 2005-1//EN"
 "http://www.daisy.org/z3986/2005/ncx-2005-1.dtd">
<ncx xmlns="http://www.daisy.org/z3986/2005/ncx/" version="2005-1">
 <head>
 </head>
        <docTitle>
               <text>{book_title}</text>
        </docTitle>
    <navMap>
        {menavPoints}
    </navMap>
</ncx>
        """.format(
            book_title=self._title,
            menavPoints="\n".join([chap.gen_ncx(book_idx) for chap in self._chapters[start:end]])
        )
        return ncx_base