OMG I love this cover! I am sure you have seen it around but seriously, look at how awesome it is. It's already got the word "monster" in the title and the cover and illustrations inside are perfect for this moody, creepy, and dark story. I love the black and white and the sort of sketchy line drawn look of the image. It actually reminds me a little of the work of Stephen Gammell whose illustrations in the Scary Stories series used to haunt me when I was a kid.
The artist on this one is Jim Kay and his stuff is fantastic. You really should check out his website to see more illustrations from A Monster Calls. Or, better yet, go out and buy it. Here is the book trailer to give you a taste of what the book is like.