On a bitter-cold day, in the December of his junior year at Harvard, Sam Masur exits a subway car and sees, amid the hordes of people waiting on the platform, Sadie Green. He calls her name. For a moment, she pretends she hasn’t heard him, but then, she turns, and a game begins: a legendary collaboration that will launch them to stardom. These friends, intimates since childhood, borrow money, beg favors, and, before even graduating college, they...