Debt is the hidden engine driving undocumented migration to the United States. So argues David Stoll in this powerful chronicle of migrants, moneylenders, and swindlers in the Guatemalan highlands, one of the locales that, collectively, are sending millions of Latin Americans north in search of higher wages. As an anthropologist, Stoll has witnessed the Ixil Mayas of Nebaj grow in numbers, run out of land, and struggle to find employment. Aid agencies have provided microcredits to turn the Nebajenses into entrepreneurs, but credit alone cannot boost productivity in crowded mountain valleys,...