Henry Mayhew wrote a comprehensive series of articles for the Morning Chronicle in the 1840s about the poor in London, interviewing hundreds of Londoners about their lives and livings. He collected and summarised his articles in three volumes in 1851, of which this is the first.
Some of the analysis has dated badly, but the raw information is fascinating: his interviews are extremely impressive both in breadth and depth. This is a key source for anyone who wants to write about — or in the voices of — 19th-century London. Well worth reading — or at least flicking through for the bits that interest you.
Available from the Internet Archive. The copy they've scanned belonged to Edward Healy Thompson; I imagine he found the sections describing Irish Catholic life in London particularly interesting.