His Excellency: George Washington is a 2004 biography of the first President of the United States, General George Washington. It was written by Joseph Ellis, a professor of History at Mount Holyoke College, who specializes in the founding fathers and the revolutionary and federalist periods.
Joseph Ellis doesn't know anything about British History. 
The context says Ellis is a professor of History so he probably knows a good bit about the history of many different countries, but it also says he "specializes in the founding fathers and revolutionary and federalist periods" so he also may only focus on American history. 