Local governments spend over 12 billion dollars annually funding the operation of 17,000 public libraries in the United States, yet we know little about their e ects. We use data describing the near-universe of public libraries to show that public library investment increases children's attendance at library events by 18%, children's checkouts of items by 21%, and library visits by 21%. Increases in library use translate into improved test scores in nearby school districts: a $1,000 or greater per-student capital investment in local public libraries increases reading test scores by 0.02 standard deviations and has no e ects on math test scores.