'Return a 3-tuple or None. The 3-tuple is of the form:
(identifier_type, identifier_value, URL).
The URL is the URL for the book identified by identifiers at this
source. identifier_type, identifier_value specify the identifier
corresponding to the URL.
This URL must be browseable to by a human using a browser. It is meant
to provide a clickable link for the user to easily visit the books page
at this source.
If no URL is found, return None. This method must be quick, and
consistent, so only implement it if it is possible to construct the URL
from a known scheme given identifiers.'

def get_book_url(self, identifiers):

    return None
