The book "Appalachian Trail Guide to Shenandoah National Park" is a must!! Published by the Potomac Appalachian Trail Club, this guide book proved ideal for both pre-planning and as a companion on the trail. My small copy is the 14th edition (2012), and I carried it in my pack's top flap for handy reference as needed. I purchase mind from the PATC.net website directly. Newer additions are available, so check the club's website before buying something old off Amazon.
The BEST Maps! Also by the Potomac Appalachian Trail Club. For Shenandoah, the maps to get are 9, 10, and 11. Order directly from the PATC. Here's the link to the PATC maps page for ordering. These are nicely made maps that weather the outdoor elements well. You spread them out on a picnic table or the ground. The three maps detail north, central, and south districts of the AT within Shenandoah NP. I always carried in my pack's top pouch the numbered section I was currently hiking. Invaluable, in my opinion.
If staying at Big Meadows, this map is handy. It shows the layout of the Lodge, amphitheater, campground, various trails, and more. The AT runs right through Big Meadows, so hikers can hop on the AT for a spell if they aren't thru-hiking the whole thing.