Colm Tóibín at The Guardian:
Part of the power of the book comes from the grim, unearthly picture it draws of London. Since London was a collection of villages in which anyone moving from a posh square to an important public building could catch a glimpse down the many side streets that housed the poor, in which privileged and pauper passed each other daily, then the novel itself gained nourishment from the friction between classes, from the closeness of the little streets to the great. In the later 19th century, a number of writers saw the startling possibilities such contrasts offered, where the London of Dickens, so sprawling, vast, and filled with drama, could be rendered as a ghostly place where substance became shadow.
In books such as Robert Louis Stevenson’s Strange Case of Dr Jekyll and Mr Hyde (1886) or Oscar Wilde’s The Picture of Dorian Gray (1891), London could become a place of secrets, a city in which many people, otherwise normal and sane, could change their form as they moved from street to street, or from drawing room to attic.
Henry James would set his novel of 19th-century terrorism in this London. The Princess Casamassima (1886) “proceeded quite directly,” he wrote, “during the first year of a long residence in London, from the habit of walking the streets.