Learn You a Haskell for Great Good! (A Beginner's Guide) by Miran Lipovača, published by No Starch Press (2011). No Starch was kind enough to send me an advance copy for review.
Haskell books for "real programmers" are still thin on the ground, being limited at the moment to Real World Haskell (2008) and possibly Programming in Haskell (2007). As its introduction states, this book is aimed at existing programmers who are currently fluent in something like Java, C++, or Python, and would like to learn Haskell.
I put my take on the traditional discussion of why you should consider learning Haskell in another blog post, so we can get on with the review here.
The hardest thing about learning Haskell with no previous functional experience is bootstrapping the strong foundation that you've long since taken for granted in your imperative language. If you don't have this strong grasp of the fundamentals, then every line of code is an invitation to get stuck on some subtle issue, and you'll never have the fluency that great work requires until you have that foundation.
This book is the best way I know to obtain the Haskell foundation you need for fluency.