I'm writing a book on Flask, the Python web framework 13 years ago
Flask is a great book topic. So just an add on, I think it is always good to see what else is out on there on the topic as a starting point. And I just purchased this book that recently came out on Python for the Web and it speaks a lot about Flask as well. So it may be beneficial to look that over and see what that author already has out there, so you could expand or hit different points. Here's the link if you are interested, http://www.realpython.com/