A GamePlayKit by Tutorials Book
I read the Swift Apprentice and most of the 2D Games. Fantastic Books!
I would like you to consider create a GameplayKit by Tutorials Book.
Its really a shame the almost none existent documentation on how to implement the GameplayKit API.
I only found a couple here on your website and one video in youtube.
Covering all the topics a not only Entities and State Machines but also Rule Systems, Desicion Trees and integration with Xcode (shown in the WWDC 2016 but not taught enough)