This is an amazing book. I highly recommend it as the first Haskell book you read. After it, you can check out Real World Haskell.
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 started reading the online version, enjoyed it so much, I went ahead and boug ht the treeware version. And it's awesome.
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. :-)