If you don't fancy getting your hands on Dead Island's "Blood Bath" DLC, or you just hate GameStop, then you can pre-order the game at Amazon for a different bonus -- a digital art book.
You can check out two sample images in the gallery if you want. Can't say I'm swayed by this, personally. If I were to pre-order it, I'd definitely go with GameStop's DLC over a book that isn't really a book. That's just me, though.