Rebecca Boggs Roberts opens Untold Power, her delightful new biography of First Lady Edith Bolling Galt Wilson, with a gripping scene that seems scripted for Hollywood. To set the stage: In the fall of 1919, after a grueling seven months in Europe negotiating the end of World War I, a bruising battle with the Senate over the treaty’s ratification, and a cross-country train tour to promote the League of Nations, President Woodrow Wilson suffered a series of strokes that left him partially paralyzed, intellectually diminished, and often incoherent. Remarkably, Edith Wilson—the president’s second wife, of just four years—along with...