I did a southbound A.T. thru-hike in 2008 (trail name "Mercury", if you were wondering), and I carried the 2007 edition of the Thru-Hiker's Companion with me (having received it as a Christmas present the previous year). With its detailed listings of trail features (shelters, campsites, water, road crossings, locations of grocery stores and towns, among others), this book was easily the most useful item of non-gear I carried. It's small and compact enough that it's not unreasonable to carry the entire thing, but it's also designed to be ripped into sections if you'd rather carry less weight. It's clear and concise, with a minimum of unnecessary detail -- what you need plus the occasional cultural note, and nothing more. The details it gives will be extremely important in resupplying, determining how much water must be carried (excepting summertime when sources begin to dry up) and where to stop each day, and just stopping off the trail for a good meal or a place to stay. The trail's well-marked enough that you don't need a map, so you'll need an idea of where things are along it, and this book does the job superbly. Beware: this isn't an A.T. how-to; get this book, but if you're a relative backpacking novice, get another book designed for that purpose too.
...but really, if you're reading this, you shouldn't get this book. :-) Get the newest edition, and don't try to save money getting a previous year's edition. My 2007 edition was usable in 2008, but I found a number of places where businesses (and occasionally shelters) along the trail no longer existed, including once when I discovered such a closure after an extra mile of walking at the end of the day. There's no reason to get an older version when a newer version is just as easy to get and will be more accurate.