To travel the Silk Road, the greatest land route on earth, is to trace the passage not only of trade and armies but also of ideas, religions, and inventions. Making his way by local bus, truck, car, donkey cart, and camel, Colin Thubron covered some seven thousand miles in eight months?out of the heart of China into the mountains of Central Asia, across northern Afghanistan and the plains of Iran into Kurdish Turkey?and explored an ancient world in modern ferment.