Children's exposure to book reading is thought to be an influential input into positive cognitive development. Yet there is little empirical research identifying whether it is reading time per se, or other factors associated with families who read, such as parental education or children's reading skill, that improves children's achievement. Using data on 4,239 children ages 0-13 of the female respondents of the 1979 National Longitudinal Survey of Youth, this study applies two different methodologies to identify the causal impact of mother-child reading time on children's achievement scores by controlling for several confounding child and family characteristics. The results show that a 1 SD increase in mother-child reading time increases children's reading achievement by 0.80 SDs.