Flexy box is a document I created with notes and demos that I used to learn the concepts of the Flexible Box layout. I referenced all the sources listed bellow.
Another really cool post that helped me with flex box is a visual guide to flexbox layout. This article has great visuals on flex box concepts and syntax.
I also referenced from the W3C spec. Yeah it can be a little overwhelming but it's where flex box is documented most accurately.CSS Flexible Box Layout Module Level 1.
The last thing I'd like to add in is a really cool web app that helped me visualize some flex box layouts. Play around with it! It's supper awesome! Flex box is 5