One can't really say what's "wrong" with it without knowing exactly what your vision was, but some of things that bother me are the cropped bottle (Heck, Guinness in a bottle bothers me; if it doesn't come out of a tap, it's not real Guinness!), the low perspective, the blank background, and the fact that it appears to be slghtly under-exposed.
I get the feeling that this was intended to be a rather 'high-brow' arsty sort of food shot, which seems completely at odds with my understanding of the book in question.
Given that the cake looks like a dark chocolate cupcake, it's very hard to link the Guinness to it effectively. What I see is a serving-suggestion image where you're recommending an iced cupcake and a bottle of stout; both good, but not neccessarily in combination. I think for this, you need to show both the finished product and the preparation; say someone pouring the Guinness into a mixing bowl with a batch of fresh cakes cooling in the background. This is a tough one...