FOUR CORNERS: History, Land and People of the Desert Southwest by Kenneth A. Brown (HarperPerennial: 372 pp., $13.50, illustrated). Diverse cultures and geological formations met in the Four Corners area long before the U.S. government divided the region among Arizona, New Mexico, Colorado and Utah. Brown's rambling overview examines the geographic and geological factors that produced the dramatic canyons, mesas, deserts and mountains. The limited water supply restricted the populations of the Native American tribes who settled there, but a combination of agribusiness and rapidly expanding tourism now threatens this fragile ecosystem. Brown warns that a sensible land-management policy is long overdue.