Incorporeality is one of the easiest to misunderstand rules in the Pathfinder Roleplaying Game - and there are a lot of good reasons for it. It's something that has worked differently in every iteration of the game so far, and it's not even the same in the different places it appears in the Pathfinder Roleplaying Game!
With What Is Incorporeal, you have quick and dirty reference for all of the incorporeality rules.
And with the price being Pay What You Want, there's no good reason not to pick it up, too.