It’s tough to say what or who was more impressive—the cocktails on the windswept lawn of Mount Vernon Estate, the guests in formal dress, the golden late afternoon light, the views of the shimmering Potomac, or the evening’s honoree, who got her undergraduate degree at Harvard, her master’s at Oxford and her PhD at Yale. Let’s say it was a draw, because all were tough acts to beat and all complemented one another.

The occasion was the eighth annual George Washington Book Prize dinner, hosted by Washington College of Chestertown, Maryland; the Gilder Lehrman Institute of American History; and the Mount Vernon Ladies’ Association, a formidable nationwide group that has kept the historic estate viable for more than 150 years.

Out of four dozen nominees and three finalists, the $50,000 prize was awarded to Maya Jasanoff, an associate professor of history at Harvard. Her book, Liberty’s Exiles: American Loyalists in the Revolutionary World, was lauded for its research, concept, perspectives and “enviable prose style and penetrating insights” by the jury that determined the prize’s finalists. It focuses on a niche piece of the American Revolution, the approximately 60,000 American “loyalists” who doubted their future in the new nation and departed US shores for exile in British territories such as Canada, Jamaica, the Bahamas, and India....