I'm glad to hear that you and your son have taken such an interest in Unity. It is a powerful game engine with so much potential.
I personally have not read up on any Unity books, but I am seeing that "Unity in Action" does come up a bit in my Google searches. I would also consider posting in the Unity forums if you haven't done so, as the Unity community may have some more in depth answers as well.
Best wishes on your future learning!