[AlexanderNull]

While Scala isn't a pure functional language there are some good resources for picking up FP related concepts with it (and it's what I recently moved over to so it's most recent for me). Scala with Cats did a pretty good job for me at describing things in relatable terms. "The Red Book" (Functional Programming in Scala) goes a bit more in depth on these concepts but has a pretty steep learning curve compared to the cats book.

Also check out anything Haskell/Eta/OCaml for server side (and probably also a bit of a steep learning curve) or if you're currently a front end developer CHECK OUT ELM NOWWWWWW. Core beginner concepts to youtube would be currying, higher order functions, and composition. Monads will come once you've mastered map/flatMap/reduce/fold chaining.