The following references will help you dive deeper into Flask (and also Django):
- Mastering Flask Web Development – Second Edition by Daniel Gaspar and Jack Stouffer (https://www.packtpub.com/web-development/mastering-flask-web-development-second-edition)
- Flask – Building Web Applications (https://www.packtpub.com/web-development/flask-building-web-applications)
- Flask By Example by Gareth Dwyer (https://www.packtpub.com/web-development/flask-example)
- Python web development: Django vs Flask in 2018 by Aaron Lazar (https://hub.packtpub.com/python-web-development-django-vs-flask-2018/)