The period 1870-1930 witnessed the emergence of the local public library as a widespread and enduring American institution. During these years, access to free community-based library services spread to a much larger share of the U.S. population, while the institutional structure of local libraries underwent a transition from largely quasi-private, voluntary associations to the tax-supported public institutions familiar today. In this paper we describe this transition, and document the expansion of public libraries and library services in the United States over these years, using data drawn from library surveys conducted by the federal Bureau of Education. We then review some causal accounts for that expansion. Exploiting cross-state and temporal variation in the data, we estimate cross-section and panel regressions to assess plausible demand and supply factors affecting the pace of library development. We consider a number of the social and economic variables that have been found to correlate with the development of secondary educational institutions, which expanded during roughly the same historical period: these include, when available, income or wealth, urbanization, ethnic composition, and in some specifications average levels of education and literacy. We also examine the effect of legal and political factors that were specific to public libraries, such as state library commissions and associations.