This paper looks at results from the 2009 Programme for International Student Assessment to examine the effects of school libraries on students' test performance, with specific focus on the average of students' family wealth in a school. The paper documents students' school library use and students' home possessions to indicate how school libraries support students and which students need that support the most. ANOVA results indicate that poor students (those whose family wealth was in the lowest third) make more use of the school library than their wealthier peers and that poor students' families are less likely to have information and cultural resources than wealthier students' families. Hierarchical regression indicates that school libraries and, more specifically, school library adequacy, as defined by the principal's perception of adequate staffing and materials, account for a small but significant portion of students' test performance variance. Finally, quantile regression indicates that the benefit of school libraries is not spread evenly, and some students gain more benefit than others. A direct implication of this research is that school libraries have the capacity to better serve poor students but will need the support of policy- and decision-makers before such a change happens.