You may want to have a look at Erlang and Elixir for Imperative Programmers (I haven’t read it, so I can’t really comment).
Personally given that even Erlang/Elixir aren’t “purely functional” I started reading Haskell Programming from First Principles - admittedly an extreme approach but I felt that my brain needed an extreme push 
(I think Erik Meijer just finally wore me down with his “learn Haskell”).
