When the most exciting producer in pop music himself puts out a CD, the thrill is in the discovery: The world may have already suspected that Justin Timberlake, Nelly Furtado, and a small army of other singers owed 75% of their fame to the stockpile of skittering beats Timbaland built and then graciously set them upon, but here’s a chance to prove it. Or it might have been in less welcoming hands. Shock Value is a far-reaching and ambitious disc; a masterpiece, even, in its own way.