If you’re interested in functional programming I’d start skimming through Scott Wlaschins site F# For fun and profit, watch some of his videos on YouTube and, get a feel for the concepts. The site is primarily about F# but the concepts are language agnostic. It will be a bit harder to do some things in C# but that’s where this book comes in. Make sure to work through the exercises and check out the source code for both them and the examples.

Incorporating the techniques and patterns in your own code will start you off in thinking functionally. Be warned though, there might be some resistance. Thinking about code if you’re coming from the regular old imperative programming style will be very different.