Of the half-dozen full-length histories of the battle of Gettysburg written over the last century, Gettysburg: The Last Invasion is the first to dive down so closely to the experience of the individual soldier, or looks so closely at the sway of politics over military decisions, or places the battle so firmly in the context of nineteenth-century military practice.
"Gettysburg: The Last Invasion" by Allen C. Guelzo
