Greene's novella of The Third Man reminds me a lot of Whit Masterton's Badge of Evil (adapted by Orson Welles for Touch of Evil). It is a pacy story, exciting, with enough to keep a reader's interest, but when contrasted with the film that followed it does not bear comparison.
This is simply a film treatment. It was a novella written by Greene to provide a plot, and characters for an original screenplay director Carol Reed wished him to write (following an earlier successful collaboration). It was never intended to be a stand alone novel. And in a fascinating introduction Greene advises the reader of the changes forced on the original screenplay in the collaboration.
In the novel the story is narrated by Major Calloway, and is reliant on other's recollections of events (notably the writer Rollo Martins). The central character (Joseph Cotten in the film) is Rollo, not Holly Martins. Rollo being an English writer of Westerns under the pen name Buck Dexter. This leads to a "comic" misunderstanding where Martins is mistaken for a great English Man of Letters, B Dexter. Never convincing the change to an American lead ejects this from the film, and allows the comedy of the literary meeting to arise from Martins championing by Calloway's sergeant in the film.
The change to an American lead in the film, and therefore the change in nationality of Harry Lime (originally to have been played by Noel Coward, but thankfully played by Orson Welles in the film) meant that an anicllary character (Cooler) became Romanian in the final film - in order to avoid upsetting American filmgoers.
Aside from the changes to character, there are one or two alterations to plot (particularly in relation to Anna).
The novella as a stand alone text is a passable entertainment, and demonstrates Greene's ability at creating quirky interesting characters, and giving a novel a sense of place and atmosphere. Vienna is wonderfully evoked (although whether this stems simply from the writing, or is recollections of a wonderful film, I cannot be certain). It does not rank alongside the great Greene entertainments, such as Our Man in Havana; and certainly cannot rate with great novels like The Power and the Glory, The Human Factor, or The Heart of the Matter.
This is little more than an interesting curiosity, an opportunity for a reader to view the rough draft of a screenplay for one of the greatest films ever made. From it we learn that Greene could not write a book that was not entertaining, but we also see just how much of a role Carol Reed, actors, and music, had in creating the final film. Film is very much a collaborative process, and this film treatment was written with that very much in mind.