Sometimes termed the bible of A.T. hiking, each year's Data Book consolidates the most basic information from 11 detailed guidebooks into a lightweight table of distances between major Appalachian Trail shelters, road-crossings, and features. It is divided according to the guidebook volumes (one state or two or, for Virginia, parts of a state) and updated each December to account for Trail relocations, new (or removed) shelters, and other changes. In addition to codes for lodging, food, water, and other essentials, the Data Book is keyed to both the individual guidebook sections and to the separate maps. Day-hikers and long-distance hikers alike rely on this volume for armchair planning as well as on-the-trail orientation. This also is the source for the ever-changing official answer to, How long is the trail? Officially for 2010, it's 2,179.1 miles.
If you want to hike the A.T., this book should prove a invaluable resource. Updated annually with changes to important features for hikers, like locations of water, campsites, shelters and distance/direction to towns along the trail, the Trail Data Book is perfect for planning. I look forward to obtaining the 2017 edition to finalize plans for my hikes this summer.
For people doing the AT I'd recommend the AT Thru-Hikers' Companion as there is more information inside; the data book is a repetition of what is written in the AT Thru-Hikers' Companion. If you solely wanna have the key data regarding towns and shelters, then this one is a good option (also lighter than the other one).