I just discovered a fantastic resource online. The Library of Congress "American Memory" website has a huge collection of Civil War maps.
I have been looking at maps of Northern Virgina to get an idea of the terrain fought over in the skirmishes that dominated the months following the First Battle of Manassas. This site is fantastic. It has several maps that were prepared for General McDowell in early 1862 in which individual farmsteads are listed and a good idea can be had as to the location of orchards and the like. If you have not browsed this site before, I highly recommend it.