Great book Chris (and RW Team). In my opinion - SceneKit never got the attention it deserves. It is already a pretty solid framework and you can go a long way with it. Quickly skimmed the book and the examples are really good and well-explained (nothing new when it comes to RW of cause With this book you can get started on SceneKit right away and learn to use it - fast.
A bonus chapter on creating 3D assets using MagicaVoxel might be a good idea - while it is fairly easy to make models in MagicaVoxel getting them into SceneKit is not as straight forward as it might sound. Getting them to scale right - and facing the right way takes some work - and will require the use of Blender or other 3D software to do it right (also - make sure the models are optimized as much as possible).
As per the usual attention and care the Ray Wenderlich team puts into all of their content, I can only expect this book to be well worth it. Looking forward to seeing what comes next!