I worked through part of the "Real World Haskell" book a few years ago, but settled on mostly using Clojure as a functional language, with some Scala also. I bought "Learn You a Haskell for Great Good!" this week and I have been enjoying the gentle approach to learning the language. Miran Lipovača did a good job writing this book. ("Real World Haskell" is also excellent.)
One thing that occurred to me is that since Clojure and Scala borrow so many good ideas from Haskell that learning some Haskell before diving into either Clojure or Scala might be a good idea.