How the Other Half Lives: Studies Among the Tenements of New York by Jacob A. Riis (1849 - 1914)

In their own words

How the Other Half Lives: Studies Among the Tenements of New York (1890) was a pioneering work of photojournalism by Jacob Riis, documenting the squalid living conditions in New York City slums in the 1880s. It served as a basis for future muckraking journalism by exposing the slums to New York City’s upper and middle class. The title of the book is a reference to a phrase of François Rabelais, who wrote in Pantagruel: "one half of the world does not know how the other half lives". (Summary by Wikipedia)