This is still in Beta, so not yet finished (and has the last few typos to be fixed), but is a great introduction.
It even covers some of the recently added features of the venerable GenServer so you get to see how to use
handle_continue which is great technique that allows a genserver to return and still carry on working on a problem.It starts with Tasks, introduces GenServers.
Then it moves onto GenStage (a system for using Backpressure to control the flow of data through an application.
Flow adds wrappers around GenStage to simplify the usage (it’s almost as easy as Streams).
The examples are mostly contained within individual chapters which makes picking up a topic much easier than other PragProg books that I have worked through.
Awaiting the B3 release!
Developer Rants 