The Data Book was invaluable to me on my thru-hike, with information about trail mileage, road crossings and distances to town, shelter and campsite locations, water sources, etc. available at a glace. If I HAD to choose between the Data Book and The Companion / or Handbook, I’d go with the Data Book. I referred to it multiple times a day. I did also carry the Thru-Hiker’s Companion, though, and often looked over the next section of that before heading in town. But I don’t recall (it’s been almost seven years now since I looked at it) that the Companion (or Handbook, for that matter) had all the water sources listed, for one thing. What the Data Book offers, along with the additional town info in either of the other two guides, would be, in my opinion, perfectly sufficient. Just don’t be surprised, though, if occasionally information in the Companion or Handbook is already outdated once you’re on the trail, even if you have the latest copy. Things change and can change suddenly in towns–ie. businesses close, rates change–and I found quite a few instances where what I read was not what turned out to be.
As far as maps, I didn’t carry them and wasn’t sorry. I did look over people’s shoulders when, in camp, they’d look at the profile of the next day or so, but just for curiosity-sake. I only veered off the trail once on the entire hike and quickly realized my mistake and knew where I’d gone wrong, no map necessary. One thing the maps did provide that neither the Data Book or my Companion did, was indication of additional water sources that sometimes paralleled the trail, so occasionally there ended up being more water available than I’d known beforehand. But that wasn’t really a big deal, since more than enough was better than less than expected.
Overall, I think the A.T. is quite forgiving of even those who set out with not much information at all (not to mention little or no backpacking experience). With usually plenty of water–with a couple of notable exceptions maybe, but still not THAT big of deal–and so much access to towns, it’s quite easy to adjust as you go and learn on the trail.
ramkitten