This paper describes a 12-week cluster randomized controlled trial that examined the efficacy of BookNook, a virtual tutoring platform focused on reading. Cohorts of first- through fourth-grade students attending six Rocketship public charter schools in Northern California were randomly assigned within grades to receive BookNook. Intent-to-Treat models indicate that students in cohorts assigned to BookNook outperformed their control-group peers by roughly 0.05 SDs. Given the substantial variability in usage rates among students enrolled in BookNook cohorts, we also leveraged Treatment-on-the-Treated approaches. These models suggest that students who completed 10 or more BookNook sessions experienced a reading advantage of 0.08 SDs, while those who completed 20 or more sessions--the recommended dosage--experienced a 0.26 SD developmental advantage.