Lilliana Mason's Uncivil Agreement: How Politics Became Our Identity is easily the best book on American politics I have read in years. I mean this in two important ways. First, the book tackles what may be the most pressing question in politics: Why has the American public become increasing polarized? The answer—that the increasing overlap between identities changes the way that citizens see themselves and others—provides a clear understanding of polarization. But this is not only an important book, it is a good book. Mason constructs a careful argument, grounded in social psychology, and each chapter in the book builds sequentially on the previous ones. The result is a book that is more than the sum of the parts and represents a major advance in the field. I lost count of the number of times where Mason demonstrates a point that clearly articulated some previously unintelligible hunch I had about politics. There are few books that make this type of contribution to a vital question in the way that Uncivil Agreement does.