Policies that reduce the costs of accessing prior knowledge (which is covered by copyrights) are becoming increasingly prominent, even though systematic empirical evidence on their effects continues to be scarce. This paper examines the effects of the 1942 Book Republication Program (BRP), which allowed US publishers to replicate science books that German publishers had copyrighted in the United States, on the production of new knowledge in mathematics and chemistry. Citations data indicate a dramatic increase in citations to BRP books after 1942 compared with Swiss books in the same fields. This increase is larger for BRP books that experienced a larger decline in price under the program. We also find that effects on citations are larger for disciplines in which knowledge production is less dependent on physical capital: Citations to BRP books increased substantially more for mathematics (which depends almost exclusively on human capital) than chemistry (which is more dependent on physical capital).