and written as an original screenplay as i recall; i'm a first edition collector and have many of Greene's books but not this one. it's a 'treatment' of the movie and i think came out about a year after.
Yes, that's correct. From Wikipedia:
"Before writing the screenplay, Greene worked out the atmosphere, characterization, and mood of the story by writing a novella. This was written purely to be used as a source text for the screenplay and was never intended to be read by the general public, although it was later published."
I read a few of Greene's books two summers ago and really enjoyed some of his lighter fair such as Our Man in Havana and The Confidential Agent. I also tried one of his more serious works, Heart of the Matter, and found it extremely depressing. Incredibly well-written, but painful to read.