A $50,000 prize has been awarded to a book about the Civil War conflict in Gettysburg. The book by Allen C. Guelzo, “Gettysburg: The Last Invasion,” received the inaugural Gugenheim-Lehrman Prize in Military History. Last year, the award was established by the Harry F. Guggenheim Foundation and Lewis E. Lehrman, a businessman and former Republican candidate for governor and co-founder of the Gilder-Lehrman Institute of American History. Five other finalists were also in the run for the award, including “The Guns at Last Light,” Rick Atkinson’s acclaimed World War II trilogy.