To improve the reading comprehension outcomes of children in high-poverty schools, policymakers need to identify reading interventions that show promise of effectiveness at scale. This study evaluated the effectiveness of a low-cost and large-scale summer reading intervention that provided comprehension lessons at the end of the school year and stimulated home-based summer reading routines with narrative and informational books. We conducted a randomized controlled trial involving 59 elementary schools, 463 classrooms, and 6,383 second and third graders and examined outcomes on the North Carolina End-of-Grade (EOG) reading comprehension test administered nine months after the intervention, in the children's third- or fourth-grade year. We found that on this delayed outcome, the treatment had a statistically significant impact on children's reading comprehension, improving performance by 0.04 SD (standard deviation) overall and 0.05 SD in high-poverty schools. We also found, in estimates from an instrumental variables analysis, that children's participation in home-based summer book reading routines improved reading comprehension. The cost-effectiveness ratio for the intervention compared favorably to existing compensatory education programs that target high-poverty schools.