Functional programming is a different way of thinking about solving problems. People say it's about thinking about "what you're trying to compute" versus "how you're computing it," but quite frankly that never made sense to me. I prefer to think about it in terms of thinking about relationships (FP) versus thinking about steps over time (procedural).

This, at least for me, brings the act of writing a specific piece of code more inline with how I think about the system as a whole. I spend less energy worrying about the current state of the world and more about composing small, predictable operations on relationships.

As for what you can read, I find it's just best to get going with something like OCaml or F# and write something that can take advantage of that paradigm in a relatively straightforward way, like a compiler or something else with a lot of graph operations. You'll learn pretty quickly what the language wants you to do.