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 effects. 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 effects on math test scores.