Date of this Version
The Sibley Field Guide to Birds of Eastern North America and The Sibley Field Guide to Birds of Western North America by David Allen Sibley (Knopf, 2003, $19.95 each).
Sibley has taken up the mantle of Roger Tory Peterson, who gave birth to the modem avian field guide in the 1930s and guided its development for more than half a century. I think that Roger would be pleased with the direction that David Sibley is taking it. The Sibley guides are so good that further editions are a dead certainty. As with the Peterson guides, I will likely buy every edition, whether or not I need it.