First Day at Gettysburg: Crisis at the Crossroads (Unabridged‪)‬

"Hassler's history will survive as our most detailed narrative of the first day's battle, examining the day's action so minutely that no succeeding historian of Gettysburg will be able to ignore it. Hassler's book has solid virtues in addition to its thoroughness of detail: It offers a persuasive argument that the first day's events largely determined the eventual outcome of the battle; Hassler displays uncommonly complete knowledge of the battlefield terrain [and] makes uniquely good use of the information that can be gleaned from the monuments and markers on the battlefield." (
American Historical Review)