Copyrights for books, news, and other types of media are a critical mechanism to encourage creativity and innovation. Yet economic analyses continue to be rare, partly due to a lack of experimental variation in modern copyright laws. This paper exploits a change in copyright laws as a result of World War II to examine the effects of copyrights on science. In 1943, the US Book Republication Program (BRP) granted US publishers temporary licenses to republish the exact content of German-owned science books. Using new data on citations, we find that this program triggered an 80 percent increase in citations to German-owned science books. This increase was driven by a significant reduction in access costs: Each 10 percent decline in the price of BRP book was associated with a 45 percent increase in citations. To investigate the mechanism by which lower book prices influence science, we collect data on library holdings across the United States. We find that lower prices helped to distribute BRP books across US libraries, including poorer institution. Analyses of the locations of citing authors further indicate that citations increased most for locations that gained access to BRP books. Results are confirmed by two alternative measures of scientific output: new PhDs in mathematics and US patents that use knowledge in BRP books.