I agree with you -- it is not easy to determine a theme for this book because it is really more of a travel guide. Such a book does not have the same sort of obvious theme that a more traditional work of scholarship would have.
I would argue that this book has a much more implied theme. The theme that McPherson seems to be trying to bring out is that this was an especially significant battle in American history. By paying so much attention to all the little details, McPherson is showing how important he thinks this battle is. He thinks that it is a microcosm of the Civil War, one that should make us think about the reasons for the Civil War and the pain and anguish that it brought to so many.
So I think that McPherson is trying to simply get us to revere the people who fought here. He wants us to honor their memory the way that Pres. Lincoln encouraged people to when he gave the Gettysburg Address.