I am currently reading this book. Currently in chapter 2. So far author's explanations have been really good. Although this book has a chapter on Elixir it's main focus is in Phoenix. He introduces enough Elixir to work with Phoenix.
A decade ago, I learned Ruby through the original Agile Web Development with Rails book. Afterwards I read Ruby-only books. I think both approaches work. The most important thing is probably keeping the learner engaged. For me, being able to create something I wanted (a webapp) quickly, was rewarding enough to keep me going.