For an understanding of the development of The Third Man from concept through novel and screenplay to film, the following are essential texts.
Greene, G., Collected Essays, (Penguin, 1969),
Greene, G., The Third Man, (New Windmill Series, 1972)
Reed, C., The Third Man (a film, 1949, script reproduced by Sight and Sound magazine, 2000)
There are numerous works on Greene's life and works, but to be recommended are Lodge's study, Graham Greene and Sharrock's Saints, Sinners and Comedians: the novels of Graham Greene.