I'm going to jump on the bandwagon and say that these are very well written - clear and concise, but they still retain authorial voice, and the examples work as intended. I found them of interest even though I work in general systems, but not games.

But, one question: in the "Subclass Sandbox" (http://gameprogrammingpatterns.com/subclass-sandbox.html) chapter, shouldn't the activate member function be public? That is, isn't that function the interface to the outside world? As written, the Superpower abstract class has no external interface.