+ Field(WLNS('coverClass'), 'cover_class', default=['default']),
+ Field('pdf-id', 'isbn_pdf', required=False),
+ Field('epub-id', 'isbn_epub', required=False),
+ Field('mobi-id', 'isbn_mobi', required=False),
+ Field('txt-id', 'isbn_txt', required=False),
+ Field('html-id', 'isbn_html', required=False),