I’m currently reading Programming Phoenix (Pragprog) where you get to follow along while creating an app (and I’m SO pleased you create an auth system as well!) and I also thoroughly enjoyed PragDave’s Elixir for Programmers (PragDave) where you get to code an app in an almost personal tutoring style.
If you prefer shorter examples of coding along, I agree with Peer about Elixir In Action (Manning) - you get to build things like GenServers and Supervisors from scratch which I really enjoyed, because it shows how things work under the hood which I believe helps give you a much better understanding of things.
IIRC, Learn Functional Programming with Elixir (Pragprog) did something similar with Maps 
Honestly though, I highly recommend every Elixir and Phoenix book (particularly those published by professional publishers like PragProg and Manning) - we are lucky to have some brilliant resources; check them out here Books and Courses and good luck 
