For the language and the framework the official tutorial/documentation are pretty good.
Also Pragmatic Programmers has very good books on both, written by the creators.
I love Phoenix but I will disagree with the OP in 2 points.
Yes probably Elixir/Phoenix will manage better thousands of concurrent connections, but if you are doing personal projects or are a start-up that condition is irrelevant in the meantime. So that only lefts us with the big established applications. That does not mean Phoenix is not good, it only means you will not notice a difference with Rails for most of your projects.
The second thing, that OP failed to mention is that the Rails ecosystem is at least an order of magnitude larger. From the gems available, job opportunities,developers available, instructional material, SOP for many tasks and so on.That is boring, but valuable.
Programming Phoenix is good. You might want to look at something Elixir specific too, I liked Programming Elixir although I've heard good things about Elixir in Action as well.
The official online documentation is pretty good too.
