We compared students from schools with an enriched school library--that is, one with a larger and more up-to-date book collection--with students from schools with a typical school library. We tested effects of an enriched school library on reading motivation, reading frequency, and academic skills. Fourth- and fifth-grade students of 14 schools with an enriched library (n = 272) were compared to fourth and fifth graders from 10 control schools (n = 411). Assignment to the experimental group was external and not determined by participants within schools. Students from schools with enriched libraries scored on average half a standard deviation higher on a standardized reading comprehension test than students from control schools. Mediation analysis revealed that for girls, this effect may have been obtained as a result of an increase in reading motivation and reading frequency. For boys, only reading frequency was a significant mediator.