Well spotted but we can't really hard code English text inside. The option would be to do something like:

class EpubWriter(object):
    DEFAULT_OPTIONS = {
       'nav_title': None,
    ...
    }


epub.write_epub('test.epub', book, {"nav_title": 'Table of Contents'})

and then use book title if nav_title is None or else use whatever was defined during the writing. This way existing apps using current behaviour where book title is used will not be broken.

Okay. I was trying to follow you instructions, and added it Default Title.
The I used if/else statement to check whether nav_title == None and exceute the conditions.

In the end I'm getting the error while executing the script that It is NameError: name 'nav_title' is not defined.
What Should I do? I'm a beginner in python.