Every now and then when it seems the world can’t get any greedier or immoral, a book comes along to remind us that the world’s always seemed spiritually bankrupt to the generations who lived through their own mad, bad times. That’s the implied premise of Southampton writer and historian Mary Cummings’ fascinating narrative about New York’s Gilded Age, which she revisits by way of one of the most bizarre murders in American history and its judicial aftermath, often called “the trial of the