If you really want a book pick one from here [0]. First one is good.

Personally I think just following the official guide [1] will give you all you need to get a taste of the language and the platform and decide if you like it or not.

If you were talking about websockets in particular I guess realistically most people use Phoenix Channels [2] that give you websockets in ten lines of code.

[0] https://elixir-lang.org/learning.html

[1] https://elixir-lang.org/getting-started/introduction.html

[2] https://hexdocs.pm/phoenix/channels.html