For this particular game, set at Antietam, I can recommend a couple great references. Foremost: "The Maps of Antietam" by Bradley M. Gottfried. Excellent breakdown of the main battles as well as Harper's Ferry and South Mountain. Very well done as he breaks things down into smaller parts of the field, using the Ezra A. Carman topographical/survey maps. These also included detailed Orders of Battle and other data. Outstanding book. He also has books on several other battles, which I also own. I highly recommend them.
The other is the "The Gettysburg Companion", while about Gettysburg, offers great detail on unit formation, weapons, deployments, etc. It's both a good common reference as well as a look at the Gettysburg battle in part and whole.