What Is The Best Source of Documentation for CoreOS Container Linux?

CoreOS is a bit confusing. In reality, most things are done using Container Linux. Therefore, to do things with CoreOS you really need the Container Linux documentation. DigitalOcean has a really good introduction to the whole concept:

An Introduction to CoreOS System Components

However, even this is a bit complex because it describes how to create a CoreOS cluster.

The definitive guide is actually very good:

CoreOS Container Linux Documentation