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.
Available at: http://works.bepress.com/paul_johnsgard/274/