On May 1, 1915, the Lusitania set sail from New York to England. The ship was the jewel of the British merchant fleet: It could carry 2,000 passengers, outrun nearly any boat on the water and had all the appointments of the finest hotels. Though World War I was raging on, few feared for the Lusitania’s safety, even as German U-boats lurked in the waters off Britain. After six days at sea, however, with land in sight, a single torpedo sank the great ship, leaving nearly 1,200 civilians dead. Erik Larson, author of “The Devil in the White City” and “In the Garden of Beasts,” tells the story of this maritime tragedy in his new book “Dead Wake: The Last Crossing of the Lusitania.”