The human butt has long been the object of all manner of obsessions. We worry over it: its size, its shape, whether or not it has cellulite on it, how it looks in a pair of jeans. But now, a new source of concern emerges: the alarming possibility that one’s butt — or at least, one’s relationship to butts generally — is racist. For this we may thank the existence of Butts: A Backstory, a new book by journalist Heather Radke. To be fair, it surely is not Radke’s intention to inculcate racial anxiety in her reader: Butts feels like...