there is also Elixir In Action (Manning) by @sasajuric

Think both of these goes deeper into OTP than any of the books you list - depending on the complexity of your api service you might not need that much “OTP” - but it’s always good to have an understanding..

also check this one out.. Elixir for Programmers (PragDave)

PoolToy: A (toy) process pool manager in Elixir 1.6 updates Chapter 7: Completing the worker-pool application with more recent Elixir features.

Having finished Elixir in Action by now, my opinion is that this is a must-have book for learning Elixir. The book is way more complete than Programming Elixir, and provides super valuable info about process registry, supervisors, ETS and much more. The book is filled with insights about the nature of BEAM and distributed applications. The only thing that was missing is how to make live upgrades/downgrades, which is briefly mentioned in Programming Elixir, and Macros. However, the author explains Macros in depth on his blog, even better than in Programming Elixir.

Now I’m in the middle of Functional Web Development with Elixir, OTP and Phoenix, then I’ll read Programming Phoenix and Programming Ecto and will give more info about what I learned when I finish them