In Darkest London is a social documentary of the East End in the 1880s.  It was originally published in 1889, as Captain Lobe:  A Story of the Salvation Army by John Law.  

Margaret Harkness, who wrote under the pen name of John Law, is considered one of the important expounders of ‘social realism’ in late 19^th century England.  Her passionate sense of justice and determined desire for social reform are paramount in all her writings; and her description of the impoverishment in the East End of London, where she lived for several years gathering first hand material about the lives and labour of the people there, is keenly observed.