Learn you a Haskell for Great Good is one of the best introductory programming language books I've ever read. I don't use Haskell very often, but after carefully reading this book, I'm now able to easily use functional code in my Python and CoffeeScript/JavaScript when I need to do so.

The online version is free, but I bought the book. It's worth it.

http://learnyouahaskell.com/