“In the peculiar world of textbook publishing, it is commonplace for states to require that their textbooks have a current date, so that parents can be assured that their children are reading up-to-the-minute information. No one, it seems, wants a history textbook that is five years old, even if the revised edition has nothing more than a tacked-on chapter about the events of the past few years and fleeting references to the most recent president. The demand for a recent publication date compels publishers to revise their history textbooks, even if only superficially, every few years. This makes textbooks very expensive to produce and minimizes competition by raising the cost of doing business.”