Virtual-History.Com is a great site for tracking down books on film noir generally and any books that refer to a certain film. For example this search link will not only return books on The Third Man, but a list of books that substantially reference that film:
Books about The Third Man:
Rob White, The Third Man, London, 2003
Books with substantial mentioning of The Third Man:
David Zinman, 50 Classic Motion Pictures, The Stuff That Dreams Are Made Of, New York, 1970
JerryVermilye, The Great British Films, Secaucus, N.J., 1978
Ann Lloyd (editor), Movies of the Forties, London, 1982
Anthony Slide, Fifty Classic Films 1932-1982, A Pictorial Record, New York, 1985
Neil Sinyard, Classic Movies, London, 1993
William Hare, Early Film Noir, Greed, Lust and Murder Hollywood Style, Jefferson, North Carolina, and London, 2003
Books with entries on The Third Man:
Michael F. Keaney, Film Noir Guide, 745 Films of the Classic Era, 1940-1959, Jefferson, North Carolina, and London, 2003
FilmsNoir.Net - all about film noir 