Read/create cached metadata dump of Gutenberg catalog.
	Returns:
		A dictionary with the following fields:
		id (int): Gutenberg identifier of text
		author (str): Last name, First name
		title (str): title of work
		subjects (list of str): list of descriptive subjects; a subject may be
			hierarchical, e.g:
			'England -- Social life and customs -- 19th century -- Fiction'
		LCC (list of str): a list of two letter Library of Congress
			Classifications, e.g., 'PS'
		language (list of str): list of two letter language codes.
		type (str): 'Text', 'Sound', ...
		formats (dict of str, str pairs): keys are MIME types, values are URLs.
		download count (int): the number of times this ebook has been
			downloaded from the Gutenberg site in the last 30 days.
	Fields that are not part of the metadata are set to None.
	http://www.gutenberg.org/wiki/Gutenberg:Help_on_Bibliographic_Record_Page

def readmetadata():
	"""Read/create cached metadata dump of Gutenberg catalog.
	Returns:
		A dictionary with the following fields:
		id (int): Gutenberg identifier of text
		author (str): Last name, First name
		title (str): title of work
		subjects (list of str): list of descriptive subjects; a subject may be
			hierarchical, e.g:
			'England -- Social life and customs -- 19th century -- Fiction'
		LCC (list of str): a list of two letter Library of Congress
			Classifications, e.g., 'PS'
		language (list of str): list of two letter language codes.
		type (str): 'Text', 'Sound', ...
		formats (dict of str, str pairs): keys are MIME types, values are URLs.
		download count (int): the number of times this ebook has been
			downloaded from the Gutenberg site in the last 30 days.
	Fields that are not part of the metadata are set to None.
	http://www.gutenberg.org/wiki/Gutenberg:Help_on_Bibliographic_Record_Page
	"""
	if os.path.exists(PICKLEFILE):
		metadata = pickle.load(gzip.open(PICKLEFILE, 'rb'))
	else:
		metadata = {}
		for xml in tqdm(getrdfdata()):
			ebook = xml.find(r'{%(pg)s}ebook' % NS)
			if ebook is None:
				continue
			result = parsemetadata(ebook)
			if result is not None:
				metadata[result['id']] = result
		pickle.dump(metadata, gzip.open(PICKLEFILE, 'wb'), protocol=-1)
	return metadata