Three recent books provoked by the Trump presidency grapple with identity and nationalism, drawing radically different conclu­sions. Jill Lepore, a Harvard historian and staff writer at the New Yorker, wishes that “nation-states didn’t already exist.” She finds them abhor­rent. Nevertheless, she allows that as long as the world is organized into nations, it is probably necessary for the United States to be one, with the caveat that ours must be a dry, creedal affair, based on a commitment to democracy and the Constitution with no ethnicity involved. Except, that is…