The present study investigates the incremental validity of the traditional books-at-home measure and selected extensions (i.e., number of children's books and number of ebooks) for explaining students' academic achievement as measured by their academic language comprehension. Using multiple linear regressions, we additionally explore the role of the source of information (i.e., whether information is given by parents or children). Based on cross-sectional data of a German sample of 2353 elementary school children from Grades 2 through 4, we found that parents' information on the number of books and children's books contributed to students' academic language comprehension over and above parental occupation and education. Children's information on the number of books did not further increase the amount of explained variance, and the effects were smaller than those for parents' information. Yet, when investigated separately, both parents' and children's information on the number of books and children's books at home predicted students' academic language comprehension and mediated the relationship between more distal structural features of socioeconomic status (i.e., parents' occupational status and education) and the outcome variable. No effect emerged for the number of ebooks. Our findings point to the robustness of the traditional books-at-home measure when used in parent questionnaires.