Well, there are several covers out there. But I assume you're referring to the one that looks like a bundle of nerves. I think it's supposed to be a chunk of poisoned nervous system, because of one element of the story line.
You might want to check out this site for some more opinions on the book. But watch out for spoilers in the review section if you don't want to know too much before you read it yourself.
http://www.librarything.com/work/1836933/covers/
