There are large gaps in reading skills by family income among school-aged children in the United States. Correlational evidence suggests that reading skills are strongly related to the amount of reading students do outside of school. Experimental evidence testing whether this relationship is causal is lacking. We report the results from a randomized evaluation of a summer reading program called Project READS, which induces students to read more during the summer by mailing ten books to them, one per week. Simple intent-to-treat estimates show that the program increased reading during the summer, and show significant effects on reading comprehension test scores in the fall for third grade girls but not for third grade boys or second graders of either gender. Analyses that take advantage of within-classroom random assignment and cross-classroom variation in treatment effects show evidence that reading more books generates increases in reading comprehension skills, particularly when students read carefully enough to be able to answer basic questions about the books they read, and particularly for girls.