MDN released a comprehensive guide to Flexbox with new and updated materials by Rachel Andrew. The guide includes 11 posts demonstrating layouts, use cases and everything you could possibly want or need to know on the topic. All of the related Flexbox properties are nicely and conveniently attached to the table of contents, making this extremely easy to use.
In this post, Rachel adds helpful thoughts and context about Flexbox. Her comment on Flexbox initially being treated as a silver bullet solution for all our layout issues struck me:
Prior to Grid shipping, Flexbox was seen as the spec to solve all of our layout problems, yet a lot of the difficulty in using Flexbox is when we try to use it to create the kind of two-dimensional layouts that Grid is designed for. Once again, we find ourselves fighting to persuade a layout method to do things it wasn’t designed to do.
Guilty as charged. I remember being so eager to ditch floats and learn a new syntax that I treated Flexbox as a square peg trying to be fit into a round hole. That definitely bit me on at least one project.
Most importantly about this guide is that it forms a sort of trifecta of reference materials on layout specifications provided by CSS: Flexbox, Grid and other Box Alignment properties.
Oh, and while we're on the topic, we have Flexbox and Grid guides right here on CSS-Tricks. You can never learn too much, right?