I've picked up some Haskell books - all via Kindle, I'm kind of tired of having hundreds of pounds of technical books to move from place to place. It's even more annoying when they become obsolete but for whatever reason I can't part with them. Especially O'Reilly books…

"Learn You a Haskell" by Miran Lipovača

I really like this book for a number of reasons: I was able to read a lot of it online for free, it's a fairly gentle introduction but gets into the nitty-gritty fairly quickly, and I like the author's sense of humor. I think the book would be better with exercises at the end of each chapter - I think someone else has pointed this out online but I'm not sure.

"Programming in Haskell" by Graham Hutton

This book is a little dated (some of the text is not Haskell 2010 compliant) but this doesn't really detract from how good the book is. The exercises are doable but challenging. 

"Real World Haskell" 

This book is said to "bridge the gap" between the introductory books and the application books. The introduction to Haskell in the first four chapters (or so) definitely blazes through and if read by itself would probably not be a sufficient start to the language. Used in conjunction with LYaH and PiH, it's fairly useful.