A recent book assembles a collection of studies on one of the great mysteries of contemporary American education: Why did national progress in narrowing the achievement gap separating African-American and white students stall from the late 1980s until 2004? "Steady Gains and Stalled Progress," published by the Russell Sage Foundation of New York City, offers no solid answers to that question. But the volume's studies do whittle down some popular explanations and point to a lineup of likely suspects. Like its influential predecessor, the Russell Sage Foundation book empirically explores various hypotheses for the test-score trends. Those possible explanations include: growing income inequality, differences in quality among teachers of black and white students, growth in racial isolation in schools, trends in parental education, and cultural differences.