AT Companion
Along with the Data Book, I carried another publication from the ATC called the Appalachian Trail Companion, or the Companion for short.
The Companion offered a great deal more detail about specific stops and nearby resources than the DataBook with its simple mileage and codes. The combination of the two greatly enhanced the hike, and gave me some encouragement -- things to look forward to: upcoming trail towns, along with the history and geography of the areas through which the Trail led me.