It"s true, Robert Utley writes, that mountain men such as "Crazy Bill" Williams and Jeremiah "Liver-Eating" Johnson were an unlearned, unwashed, drunk, and violent bunch who tore a bloody swath across the then-unconquered American West from the 1810s to the 1840s. Yet their travels across deserts and plains and over high mountains yielded a huge body of geographical knowledge that would enable American pioneers to cross the Mississippi and traverse the continent in relative security. Utley, a historian with a fluent narrative style, tells the stories of hard-fighting men like Jim Bridger, Benjamin Bonneville, Kit Carson, and Joseph Walker, whose names now figure prominently on maps of the region but are otherwise little remembered.