Just purchased the v0.21. I’m very happy with it; it’s been a great resource!
It would be helpful to have a section that differentiates the many different frameworks Apple offers for 3D programming, and offers an opinion of who Metal is for.
For example, SceneKit, GLKit, MetalKit, ModelIO, and Metal all solve many of the same problems (and offer abstractions to one another), but differ in their intent. Case in point: both MetalKit and ModelIO APIs are referenced in sample code in the opening tutorials.