html_file = models.FileField(_('HTML file'), upload_to=book_upload_path('html'), blank=True)
pdf_file = models.FileField(_('PDF file'), upload_to=book_upload_path('pdf'), blank=True)
epub_file = models.FileField(_('EPUB file'), upload_to=book_upload_path('epub'), blank=True)
html_file = models.FileField(_('HTML file'), upload_to=book_upload_path('html'), blank=True)
pdf_file = models.FileField(_('PDF file'), upload_to=book_upload_path('pdf'), blank=True)
epub_file = models.FileField(_('EPUB file'), upload_to=book_upload_path('epub'), blank=True)
parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
objects = models.Manager()
parent = models.ForeignKey('self', blank=True, null=True, related_name='children')
objects = models.Manager()