Jack London 
The People of the Abyss [EPUB ebook] 

The People of the Abyss is a book by Jack London made up of his firsthand account of life in the East End of London. The central theme of London’s investigative narrative is the abject physical and moral degradation of the residents of the East End, which is metaphorically called the “abyss ”.
When London wrote The People of the Abyss, the phrase ’the Abyss, ’ with its hellish connotation, was in wide use to refer to the life of the urban poor. … In the 1930s, he began disguising himself as a derelict and made tramping expeditions into the poor section of London.