I'd heard about this book for years (I know Neil Gaiman mentioned it being useful in writing Neverwhere) but only recently got a copy now that it's on Kindle. To quote the wiki on it "In the 1840s he observed, documented and described the state of working people in London for a series of articles in a newspaper, the Morning Chronicle, that were later compiled into book form."
It's party sociological study of the working poor in London and part compilation of true short stories of their lives. He goes into detail on just about every job a poor person in London could have from dustman to mudlark and more. He describes what they wear and eat, how they talk and where they sleep, where they're from and what they do for entertainment (lots of pubs, tobacco smoking, melodramas, punch and judy shows and coffee houses).
I've learned so much and I've just barely started, and it's so engagingly written it's almost like a novel. Just fascinating stuff and perfect for the Dickens/Dickens Fair fans on your Christmas list!