The purpose of this paper is to analyse how public libraries have been conceptualized and measured in official Spanish statistics over the past 15 years. This allows us to reflect on the transformation of the very nature and aims of libraries during the first 15 years of the 21st century. We examine the main official statistical source of information on cultural participation, the Survey on Cultural Habits and Practices in Spain, and track the impact of social change and digitization on library services. By analysing the 2002–2003, 2006–2007, 2010–2011 and 2014–2015 surveys, we monitor changes in the questionnaires and in the derived indicators, and relate the findings to general trends and challenges for public libraries in contemporary societies.