No woman, according to New York Mayor Jimmy Walker, was ever ruined by a book. But Christopher B. Krebs, a classics professor at Harvard, makes a strong case that an early ethnological monograph, written in the first century in Latin by the Roman historian Tacitus, may have warped the cultural identity of an entire nation. In my old Penguin translation, “Germania” — “On Germany” — runs fewer than 40 pages, but, like other comparably short documents, such as the Declaration of Independence and “The Communist Manifesto,” its influence has been earthshaking.