'return a dict containing all the custom field metadata associated with
the book.'

def get_all_user_metadata(self, make_copy):

    _data = object.__getattribute__(self, '_data')
    user_metadata = _data['user_metadata']
    if (not make_copy):
        return user_metadata
    res = {}
    for k in user_metadata:
        res[k] = copy.deepcopy(user_metadata[k])
    return res
