Probably the place I've found most of my information about the trail. The forums are golden for becoming familiar with the trail and all its nuances. I mean all of them. People will debate anything and everything, from what constitutes a thru-hiker to the benefits of down over synthetic fill (and vice-versa, of course).
Appalachian Trail Conservancy
The ATC, as it is known, is the umbrella organization responsible for maintaining and promoting the Appalachian Trail. The link above will take you directly to their "About the Trail" page, but they've got some good info spread all over their site. A better place to start than Whiteblaze, actually.
AWOL on the Appalachian Trail by David Miller
Software engineer David Miller's account of quitting his job in Titusville, Florida to pursue an A.T. thru-hike. A very honest account of the difficulties and considerations that go in to this epic undertaking, this was the first Appalachian Trail book I read. There are moments when you are reminded he is a software engineer and not a writer by training, but this is still one of the most enjoyable books about the Appalachian Trail.
A Walk in the Woods by Bill Bryson
Definitely the funniest book that exists about the Appalachian Trail (is that a hard distinction to earn?), Bryson's true story of hiking along with a recovering alcoholic is a short read and not one to be missed. Some complain that it's "fluffed" with details about the culture and history surrounding the trail, clouding the story of a thru-hike attempt, but I welcome the added details since Bryson's writing is so hilariously cynical.
The Barefoot Sisters Southbound by Lucy and Susan Letcher
Two sisters, 25 and 21, go through with a promise they made to each other a few years earlier to thru-hike the A.T. As the title suggests, they make this difficult task even more challenging by hiking barefoot. In spite the temptation to fixate on the fact that they are unshod, this book is primarily about the trail, not about how they approached it. I enjoyed how well this book conveys the emotional aspects of a thru-hike. It's a plus that it's the only book I've read about someone hiking the same direction I am, from Maine to Georgia.
The A.T. Guide 2013 by David Miller
Compiled by David Miller (author of "AWOL on the Appalachian Trail"), this is the resource for hiking the trail. I don't mean before, but as you are actually hiking. It is essentially a mile-by-mile list of points of interest, such as campsites, shelters, water sources, road crossings, nearby towns. It also has an elevation profile laid out under these amenities. This isn't something to read for pleasure to tide you over to the trail, but rather a guide to bring along on your hike. It comes in Northbound and Southbound editions.