David Goodman is a writer and fellow music lover who basically covers the gamut of everything alt-country in his second addition encyclopedia of our beloved genre, Modern Twang. Just to test him, I looked up a sampling of alt-country artists to see if he included them. On nearly every instance he has, with a few minor exceptions. But it is a comprehensive volume with loads of information on just about everyone you could possibly imagine.
The sheer magnitude of this effort cannot be overstated. Not so much for all the artist information (which typically requires a staff of people researching all the details), but just the concept of wading through thousands of artists and determinging which ones qualify for the genre. I'd say this is a definitive guide, although I detected a slant for southern music (Goodman lives in Austin). So, for example, the write-ups on Robert Earl Keen and Butch Hancock are quite lengthy, but Canadian-based Blue Rodeo gets only a half-column.
It's just a bit of a stretch to include people like Bob Dylan and Johnny Cash, but then I probably would have done the same. Not that they are alt-country, but they deserve mention in any compedium dealing with country music.
The ultimate test of a book like this is: will you read it? The answer is an unequivicable yes. The reason has more to do with the genre than anything else. It's difficult to determine whether a given artist really fits the genre, so before you buy it's handy to check out Goodman's write-up. I may be considering a new band that everyone says is alt-country, but if the write-up says its honky-tonk I will probably re-consider.
Whether a book like this is a better reference than the internet is really debatable. Certainly if you're no where near a computer it's very handy. Often I can check allmusic.com for some artist info faster than any hard-copy reference, but then AMG doesn't quite get the genre most of the time.
In general, I highly recommend the book as a definitive source and an obvious extra reference for the diehard fan.