A penniless Danish immigrant who slept on tombstones and begged for scraps, Jacob Riis clawed his way up to become a police reporter on New York's notorious Mulberry Street. Armed with a new German invention called flash powder, he raided pitch-black tenements at night to photograph a world the wealthy never had to see, essentially inventing modern investigative journalism.

His 1890 book How the Other Half Lives, powered by the new halftone printing process, put undeniable photographic reality in readers' hands and caught the attention of Theodore Roosevelt. Yet his crusade carried deep blind spots, and critics later argued his reforms destroyed the very economic ladder immigrants were using to escape poverty.

• How childhood poverty and a brutal night in a police lodging house shaped Riis's mission
• The dangerous mechanics  ...