I don’t know if Failbetters used it, but here is something for everyone. London Labour and the London Poor by Henry Mayhew is a great and just huge (4 volumes, 10 years ago or so published abridged to 1 volume - still enormous) encyclopedia of the life and manners of London as it was in the 1840s. It has interviews, statistics, wages, descriptions of various jobs, situation of minorities like the Irish, and more. A lot of this information should still be relevant in the late 19th century.
The Wikipedia article opens with a striking portrait of Jack Black, "Her Majesty’s ratcatcher," one of the people Mayhew interviewed about their trade. Download links are on the bottom: https://en.wikipedia.org/wiki/London_Labour_and_the_London_Poor
I can also support this recommendation. Following me finding out about Fallen London I actually bought one of Mayhew’s books - his fourth volume, in fact, the one specifically dealing with the criminal underworld during the victorian period (amazon link attached below). A fascinating read, with many lively descriptions and interviews inbetween the more dreary statistics. Highly recommended to people who want to delve deep into victoriana.
I have a copy of Liza Picard’s Dr. Johnson’s London left over from my college days. It’s a century too early for Fallen London, but still has some useful information.
Sorry to bring this old thread back up, but I found something that is very relevant to this discussion: the 1899 Bartholomew Fire Brigade map of London - so far the best online map for a victorian London I have found