the bible: http://blog.miguelgrinberg.com/post/the-flask-mega-tutorial-...

The official docs http://flask.pocoo.org/docs/0.10/ are also very nice although not full featured and integrated as the mega tutorial.

Seconded — Miguel's blog is terrific! Major chunks of my (very simple) website's code bear a suspicious resemblance to his examples.

I think many people's flask personal projects resemble his examples in some way ;) He covers just about everything a modern web app needs