Books
London Under: The Secret History Beneath the Streets by Peter Ackroyd (Notes from Book)
Underground London: Travels Beneath the City Streets by Stephen Smith
London: A Life in Maps by Peter Whitfield (Online Exhibition)
London: The Biography by Peter Ackroyd
London Labour and the London Poor by Henry Mayhew (Context) (Full Text)
The Art of War by Sun Tzu (Full Text)
Movies
The Fisher King (another movie where the homeless and the legendary meet)
Dark Days: A Documentary about teenagers living in the New York Subway (WATCH IT HERE)
Websites
The Streets of London: A Campaign for Homelessness Awareness. Great resource for the causes of homelessness, including testimonials.
TV Tropes: Neverwhere: Take it with a grain of salt b/c it is crowd-sourced and does not list their sources, but there are really fun bits of trivia, insight, and cultural parallels. Just double check the info if you decide to use it!
Versions of Neverwhere