Mackie is ever-provocative, just see his 'Miracle of Theism' for proof, and here he attacks morality. Our every-day moral codes, he argues, are an 'error theory' based on the presumption of moral facts which, he persuasively argues, don't exist. His refutation of such facts is based on their metaphysical 'queerness' and the observation of cultural relativity. I can't say whether he's 'right', but if you're interested in the objectivity (or otherwise) of moral standards, this is a recommended read.