This was a tough book. It was sort of first-hand sociology. Jack London decided to live in London's East End, as one of the down-and-outs there. This was back around 1900 (the book came out in 1902). His cover was that he was a stranded seaman. Once he dressed up shabbily enough, no one doubted him.
The book was essentially written in two parts. In the first half or so, London relates his experiences of living on the streets, visiting the homes of the destitute, getting in line for soup kitchens and work houses and so forth. Then in the second half, he discusses more the numbers of people involved, how 55% of the children born into extreme poverty in the East End die by age 5, how the British Empire is exporting the strong and abled out into their empire and away from England. As a result, England is becoming non-competitive on the world stage.
It's kind of the same problem as we have a century later on, the 1% at the top of the economic ladder are plundering the work and wages of the 50% at the bottom of the economic structure, and then blaming their victims for their being stunted and starved.
One interesting feature, to me, was that they kept wandering along Mile End Road. For a year, back in the dark ages, I worked at an establishment on Mile End Road: Queen Mary College, a part of the University of London. The section of Mile End Road along which I scurried daily, wasn't so bleak as the parts London trod some three quarters of a century earlier.