To elaborate on why you shouldn't read (at least the online version of) Real World Haskell first: It has a lot of important topics that it covers that LYAH does not cover (like how to use cabal their package manager). But, it's not as good of a tutorial on the language and functional programming. It glosses over very complex topics, goes into a lot of depth on details that are not so important and worst of all, gives you some exercises that you aren't capable of answering yet. I tried learning Haskell three times from that book and gave up because it just killed my confidence.
Then I discovered LYAH. It explains things very simply and at a very good pace. If you want to learn Haskell and/or functional programming, I can't recommend it more. Imagine one of the Head First books without all the corny. The only thing I wish it had was exercises.
I felt so loved as a customer of No Starch Press. They sent me the book, let me download the ebook in three free formats (no DRM), plus I got a bunch of freebie stickers.
The book itself is also quite remarkable to me in that I can read it and understand Haskell without needing a computer. I am used to reading mathematics books, and while I don't think Haskell is a programming language for all mathematicians as is so often advertised, the language does have a certain appeal to it in that you can treat its programs as abstract exercises that you can do independently of a machine. The statelessness of the whole thing doesn't need me typing stuff into a machine to see what a particular line of code. This makes it great for offline reading, away from the computer.
So yeah, get the treeware version. Support your favourite authors. :-)