In reviews of Mexican restaurants across St. Louis, you’ll find numerous mentions of “authenticity.” But what makes a Mexican dish, like the taco, truly authentic? For Ignacio M. Sánchez Prado, that’s asking the wrong question. In his new book, “Taco,” Sánchez Prado challenges the idea of authentic food by exploring how migration and modernity have shaped the taco from its origins as a working-class staple in Mexico to a transnational icon that comes in many different textures and flavors.