I bought the ethical supermarket shopping book that is made by the peeps that Leasha linked to. It's really handy because it's handbag sized. It only costs $5. They explain why all the companies are listed so if the issue isn't one for you then there's no need to boycott them.
They also have a list of the worst numbers which is handy because it saves me from having to dig around in my wallet for my little yellow list.