Not currently reading it but since Gore Vidal just died I'm reminded of his novel, "Creation," set in 500 BC. It's the fictional account of a Persian diplomat at the height of Persian empire (Darius, Xerxes, Artaxerxes), who makes extended visits to other power centres of the era (India, China, Greece). The era is interesting because it involved a "deepening" of civilisation -- centralised state power was being built and consolidated in India and China, and thinkers and mystics such as the pre-Socratics, Buddha, Confucius, and Zoroaster were coterminous with the age.