The Modern Guide to OAuth
I know what you are thinking, is this really another guide to OAuth 2.0?
Well, yes and no. This guide is different than most of the others out there because it covers all of the ways that we actually use OAuth. It also covers all of the details you need to be an OAuth expert without reading all the specifications or writing your own OAuth server. This document is based on hundreds of conversations and client implementations as well as our experience building FusionAuth, an OAuth server which has been downloaded over a million times.
Source: The Modern Guide to OAuth, an article by Brian Pontarelli and Dan Moore.