Trust no one. Believe nothing. Whenever you imagine the worst possible outcome, rest assured the final result will be worse.
Paranoia as art had been around for most of the 20th century; in The Third Man Graham Greene transformed it into entertainment, a mass-market hit on its own which connected to the unsettled postwar zeitgeist. A seminal work, unquestionably; but how good, really?
As a movie, which is how it is best remembered, The Third Man holds up quite well with a striking visual approach and engaging performances. As a book, what you get is a lean, well-paced mystery story enveloped in postwar atmosphere and European ennui.