Jack London 
The People of the Abyss [EPUB ebook] 

The experiences related in this volume fell to me in the summer of 1902. I went down into the underworld of London with an attitude of mind which I may best liken to that of the explorer. I was open to be convinced by the evidence of my eyes, rather than by the teachings of those who had not seen, or by the words of those who had seen and gone before. Further, I took with me certain simple criteria with which to measure the life of the underworld. That which made for more life, for physical and spiritual health, was good; that which made for less life, which hurt, and dwarfed, and distorted life, was bad.