From what I remember when I went back to Manassas in 2004, much of the Department of the Interior owns the land so the battlefield has not been built on. Because Gettysburg receives most of the attention, the battlefields in Maryland and Virginia don't receive as much attention. That's actually a good thing because it's almost like you have the entire battlefield to yourself.
If you're going in early April to Gettysburg, that's perfect timing. Tourist season has not started yet and once again it won't be crowded. Great thing about that battlefield is they are restoring it (as best they can from the maps that have and with the land they own) to its 1863 condition. Getting rid trees where they did not exist and planting new ones. I can guarantee you that you'll have to go back because there is just too much to see in so short a period of time. It really is a neat battlefield and the town is probably one of the most unique i have ever been to. It really is a time travel to the past.
I do have some books on other battles besides Gettysburg but the majority focus on that battle. And all really focus on the war in the east. I need to expand my collection for the war in the western theater (where some argue the war was really won).
Stephen Sears has three books. Landscape Turned Red (about the battle of Antietam. Also two books called Chancellorsville and Gettysburg
Noah Andre Trudeau has one called Gettysburg: A Testing of Courage
Earl Hess's Pickett's Charge The Last Attack at Gettysburg
Lastly if you want to get really into the battle and here stories of the common soldier look up Harry Pfanz's three books, The First Day, The Second Day, and Culp's Hill and Cemetery Hill.
That should be enough reading material to keep you occupied. Also if you need advice on where to stay if you spend more than a day, there are some great places.