John Forbes Nash, Jr., a prodigy and legend by the age of thirty, dazzled the mathematical world by solving a series of deep problems deemed “impossible” by other mathematicians. But at the height of his fame, Nash suffered a catastrophic mental breakdown and began a harrowing descent into insanity, resigning his post at MIT, slipping into a series of bizarre delusions, and eventually becoming a dreamy, ghostlike figure at Princeton, scrawling...