Good introductions to concurrent programming?

I've done a small amount of occasional threading work in C#/Java/Python at various times, but not a huge amount or anything really significant. I keep coming across languages that take a new approach to concurrency - Go, Elixir and even Pony look really interesting. But I'm struggling to really understand what the motivating examples are for their approaches to concurrency/what the tradeoffs are without a solid understanding of the sorts of concurrent programs that are written.

It seems like there's a huge amount of work going on that it'd be useful career-wise to understand and be able to engage with, but I'm not really sure where to start. Does anybody have any recommendations? I think material that approached Go or Elixir from a very practical application-first perspective would probably be really useful, but I'm open to other suggestions too!