July 2 and July 3 1863 seem to get their just due in several books which means that July 1st often gets overlooked. Historian Harry Pfanz has written an excellent book that focuses on the first day of the battle but its recent publication overshadows David G. Martin's 1995 publication Gettysburg July 1. Although Pranz does an excellent job (he always does) Martin's book details the battle on July 1st in greater detail and provides even more insight on the combat. A review on Amazon.com accurately describes the book as "Martin's Gettysburg, July 1 is now the standard source for the first day's fight." -- Civil War Regiments.
Perhaps the best thing about this book is the way that Martin is able to cover the micro and macro parts of this epic contest that began early on July 1st. Amongst its 736 pages are three extra chapters which provide the reader with the usual Order of Battle and two other interesting additions. Appendex 2 is an outstanding account of the topography of the first day's battlefield and of the road network surrounding Gettysburg. The third appendex is a brief but good overview of the weather leading up to and during the battle. Both Appendex 2 and 3 are a must for any Gettysburg buff.
The maps of this book are unique. Most Gettysburg books use the same maps but Marting goes a step further. The maps are of two kinds: a "situation" map showing the position of the troops at a given point in time and an "action" map focusing on an incident of the battle. Perhaps the biggest reason that so many authors have avoided writing a book that just focuses on day one was the fateful decision of Richard Ewell to avoid any further entaglements with the Federals after they had been repulsed to Cemetery Hill/Culps Hill. I don't want to reveal everything that Martin talks about but his analysis/opinions shift the blame off Ewell's shoulders. I won't say where he places it but I am sure that you can take a guess. I highly suggest that you read this book for yourself and make your own descision. Again it isn't as recent as Pfanz's book but is superior to that work in so many ways. Highly recommended.