America’s last Gilded Age had its “List of 400”: the people said to be worthy enough, or at least rich enough, to climb the pinnacles of high society. Today, in an age of affluence not even the Astors and Vanderbilts might have imagined, there is something closer to a List of 55. Its members are so rich that, in rarefied corners of Wall Street, they seem less like actual people than vast investment empires.