From frequent Atlantic contributor Langewiesche--a former pilot who worked the Texas-Mexico border--a terse, clear, tough- minded account of life on both sides of the line. (The title-- jargon used by customs officials on Arizona's O'odham Indian reservation--means searching for and reading the tracks of immigrants and drug-runners.) Langewiesche begins and ends his journey in the tense little Texan border town of Marfa (named, improbably, for a character in The Brothers Karamazov), where he lived for years. In between, he travels the line from Tijuana in the west all the way east to Brownsville/Matamoros on the Gulf of Mexico. He describes the waves of immigrants coming across to San Diego; the futile efforts of the Immigration and Naturalization Service (INS) to slow the traffic north; INS racism and brutality; the corruption, on both sides of the border, of law-enforcement officials protecting and profiting from the drug trade; the horrendous working and living conditions in Juarez and Matamoros as US business invests heavily in maquilas, the assembly plants where Mexico workers make less in a day than US workers make in an hour. Langewiesche sketches memorable portraits of unsung heroes and saints, Mexican and American, working to protect the rights of ``illegales'' in California or of fellow workers--mostly women, mostly young--in the maquilas. A little more Mexican history and culture, a greater sense of what's destroyed back home when Oaxaca villagers pick up and move north, would have given tragic depth to Langewiesche's report. Still, the author shows us the appalling human reality behind business-page slogans and shibboleths--NAFTA, the global economy, the free market--and he makes the border itself look as arbitrary, strange, and inevitable as the Berlin Wall in its day. And of equal geopolitical significance. Compassionate, risk-taking reporting: timely and valuable.