Novelist John Baker puts in a plug for this site in his own blog.
He also recommends a neglected book not yet listed here: Arthur Morrison’s The Hole in the Wall. Morrison, a novelist and short-story writer, is most often remembered for a series featuring the detective Martin Hewitt, but before that, he wrote several grim and violent books about life in the London slums. Tales of Mean Streets and A Child of the Jago are still in print, and you can find one of his Martin Hewitt collections at Project Gutenberg.
There’s also a free, downloadable version of “Tales of Mean Streets” available at manybooks.net — http://manybooks.net/titles/morrisonaother07tales_of_mean_streets.html