Elixir in Action is a bit outdated, no?
Some of the elixir bits are now out of date. (eg. it still has a section on Records)
But it touched on a lot of OTP topics that were missing from other places.
(this was before Benjamin Tan's book was finished)
At the time I remember Elixir in Action really made me finally understand the OTP concepts.
For comparison, though, Dave Thomas' "Programming Elixir" doesn't even mention them.