I bought the book and I can honestly recommend it (even though it's not even finished, yet). There is one bitter pill for me, though, and that's the amount of redundancy in the author's manner of writing ("chaining/adding/composing together", for example, where the "together" is entirely superfluous, and he sometimes manages to even use double redundancies, which always makes me cringe a bit).
Yes, it's nothing serious, it's just that excessive redundancy in language is a major pet peeve on mine, all the more when it's coming from a (really good) programmer, who should give examples in keeping it DRY in the first place.
Redundancies aside, the book is excellent and really explains important concepts, instead of just hinting at them and then doing things that have been done, talked about and explained a hundred times. The application is fun to build and the beautiful decoupling is also something that doesn't have enough space in currently available Books and Tutorials about Elixir, OTP and Phoenix.
Give it a try!