Ash Furrow’s forthcoming book looks good:
Ever wanted to learn about functional programming, but didn't know where to start? Grab this book to learn about applied, real-world use of functional reactive programming on iOS using ReactiveCocoa.
Update (2013-10-29): Drew McCormack:
And it is a good introduction. It’s not dauntingly long, so if you have a passing interest, you can easily read it in a few hours without having to commit to a whole new career. I recommend you take a look if FP is something that you have wondered about.
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