In September 2011, Occupy Wall Street activists took over New York's Zuccotti Park. Within a matter of weeks, the encampment had become a tiny model of a robust city, with its own kitchen, first aid station, childcare services-and a library of several thousand physical books. Since that time, social movements around the world, from Nuit Debout in Paris to Gezi Park in Istanbul, have built temporary libraries alongside their protests. While these libraries...