To Antietam Creek: The Maryland Campaign of 1862 by D. Scott Hartwig

This is what I'm reading at present and for those of you who have an interest in the subject of military history, I highly recommend giving it a read. It's a thick book and only the first volume of a two volume study on the Maryland Campaign, but it's well worth the time and effort. The second and final volume is set to be published in August and the first volume is about 650 pages (not including appendices and indexing) so if you start now you should be done in time for the final volume's release.