WHEN PRESIDENT Abraham Lincoln was assassinated 150 years ago this spring, it was easy for his shocked supporters to imagine that everyone, everywhere, was grieving alongside them. As one wrote to her brother, she was comforted by the “universal feeling of one sorrow that overcame all.”

But that nation was made up of millions of people, and in fact, as a new book demonstrates, they were anything but unified. Lincoln was shot by Confederate sympathizer John Wilkes Booth on April 14, 1865, just five days after the South surrendered the Civil War, and when he died the next day, it did not magically heal those national wounds. Historian Martha Hodes’s new book, “Mourning Lincoln,” depicts a fractious, raucous country in the wake of its leader’s assassination. Drawing from hundreds of letters and diaries from all over the country, she illustrates a much more complex portrait than one of a somber “nation in mourning.”

Others were filled with fear: Thousands of black residents of Washington, D.C., gathered at the White House as news of the shooting first spread, with some wondering if slavery would be reinstated. Still other responses were simply mundane. In New Orleans, a woman complained that she couldn’t go hat shopping because the stores had closed in response to the death.

Hodes opens each chapter with the reactions of three protagonists—a wealthy abolitionist couple in Salem and a die-hard Confederate in Jacksonville, Fla. Sarah and Albert Browne wrote frequent letters to each other while Albert traveled through the South working for the Treasury Department during the war. Rodney Dorman, a lawyer furious about the South’s defeat, kept journals that ran to thousands of pages. (Dorman, too, was a Massachusetts native, but he would later deny it, telling a census-taker he was born in South Carolina.) While Sarah Browne lamented, “No words could express enough of horror and grief,” Dorman praised assassin Booth as “a great public benefactor.”

Hodes spoke to Ideas from her home in Manhattan, where she is a professor of history at New York University. ...