A small book, tucked away in innumerable back-packs and car pockets, ever ready to hand for perhaps the majority of birders in the United States, is quite likely to be Roger Tory Peterson's A Field Guide to the Birds, first published in 1934, revised and reissued several times, and still a must-have for many bird-watchers, serious or otherwise. In 1941 it was joined by a companion volume, A Field Guide to Western Birds. Together, the two guides (the first rejected by four publishers before Houghton Mifflin finally accepted it) have sold on the order of seven million copies by 1997. The critic William Zinser suggested that Peterson's Field Guide was "the single most revolutionary development in American birding." He was the first to introduce simplified, comparative drawings and to point out key field marks (distinguishing characteristics) that help identification in the...