This novella by Graham Greene was written with the sole purpose of being adapted into a screenplay. In the book's preface, Greene states that "a film depends on more than plot, on a certain measure of characterization, on mood and atmosphere; and these seem to me almost impossible to capture for the first time in the dull shorthand of a script." Which is an interesting observation, though I'm sure that there have been plenty of incredible films made since then without being written out in novel form first. Regardless, The Third Man is a classic film, full of suspense and intrigue and atmosphere, so I suppose Greene's methods worked for him. As a novel it is not Greene's strongest (it wasn't really intended to be a novel in the first place), but worth reading if you like the film (I'd start with that first though).