So far, I agree with what everyone has posted. I would add "Gettysburg: The Story of the Battle With Maps" by Stackpole books. It looks like it doesn't have much to it, but it's soft cover, and it really does have good maps for easy access. And you won't feel bad when you get it muddy or torn up because it's not expensive.
Also, not a book, but I've downloaded multiple maps of the monuments from the Stone Sentinels website, and saved them as photos so I can access them on my phone or iPad, because on the battlefield you don't often have good internet access. For a book that locates monuments I use "Guide to Gettysburg Monuments" by Tom Huntington. It's small, and really good, and gives GPS coordinates, as well.