I have finally finished working through the example in the book.
https://github.com/chriseyre2000/mastery ( 94529b1)
This is a great introduction to OTP covering some details that I have not seen elsewhere. In particular the ability to import an OTP project and not have it auto start will be useful elsewhere.
The last few chapters do seem rushed, and the example code does not quite match what you get when it is typed in. However it is a great example of building a domain model in Elixir and then adding on the external links later. This would count as Hexagonal Architecture.
This is more of a training course that a book. You would have trouble simply reading through without working on the exercise. This does take a significant amount of time.