'Upload a list of books to the device. If a file already
exists on the device, it should be replaced.
This method should raise a :class:`FreeSpaceError` if there is not enough
free space on the device. The text of the FreeSpaceError must contain the
word "card" if ``on_card`` is not None otherwise it must contain the word "memory".
:param files: A list of paths
:param names: A list of file names that the books should have
once uploaded to the device. len(names) == len(files)
:param metadata: If not None, it is a list of :class:`Metadata` objects.
The idea is to use the metadata to determine where on the device to
put the book. len(metadata) == len(files). Apart from the regular
cover (path to cover), there may also be a thumbnail attribute, which should
be used in preference. The thumbnail attribute is of the form
(width, height, cover_data as jpeg).
:return: A list of 3-element tuples. The list is meant to be passed
to :meth:`add_books_to_metadata`.'

def upload_books(self, files, names, on_card=None, end_session=True, metadata=None):

    raise NotImplementedError()
