Frontiersman Joseph Walker has eluded biographers, Gilbert suggests interestingly, because he lived exceptionally long (1798-1876), did several different things, and left little record--out of a ""unique and, it seems, principled reticence."" What Gilbert has done, ingeniously and fruitfully, is to expand Walker's life into a de facto chronicle, even a metaphor, of the westering movement--supplying in historical detail what he lacks in personal detail. Many of the pioneers, including the Walkers, were Scotch-Irish: Gilbert explores the Old-World roots of their readiness to challenge the wilderness--where traits ""which gentler Europeans regarded as weaknesses and vices turned out to be strengths and virtues"" (and others adjusted to their norms). The Walkers themselves emerge as ""professional pioneers"": at Fort Osage on the Missouri, in still-forbidden Indian territory, ""it is likely that they became in 1819 what their grandparents had been in the Creek Nation in 1733--that is, the most westerly permanent settlers of the United States."" Joe Walker headed southwest--probably becoming one of the ""shadowy"" Taos trappers. (In 1825 he led the government field survey that marked the Santa Fe trail.) He was one of the founders and the first sheriff of Independence, Mo. (occasioning, characteristically, a discussion of early Western law enforcement). In 1832 (Gilbert speculates), when that job had become routine, and both the Santa Fe trade and trapping had turned into mercantile enterprises, he joined Army captain Bonneville's expedition to the Rockies as ""field commander""; and in 1833, led the first party through the Sierras to the Pacific. During the succeeding years, he lived among the Indians of the Great Basin, taking an Indian wife; and in 1843 he guided the first wagon train to reach California. In these and still later endeavors (involving Fremont and famous others), Gilbert sees not Scotch-Irish ""hubris"" (or, explicitly, a Reagan saber charge) but an ""extraordinary ability to get people to cooperate for the common good."" To propound this thesis, Gilbert may have assigned Walker a more sterling character than he can substantiate--but the text is a solidly and engrossingly detailed reconstruction based on assiduous research.