I watched the movie but I haven't read the book of it. I would if given the opportunity, but I suppose it hasn't been on sale or anything yet.
Anyway, the book is by Graham Greene and it centers around the early years of American involvement in Vietnam. There's a British reporter in Saigon who has a native girlfriend and is generally pretty comfortable hanging around in the city. Then an American shows up who naively thinks he can change things and drive out the commies. And he starts to compete for the British guy's girl too.
That's what happens in the movie at least, though I'm sure there's some stuff cut out. The CIA is supposed to represent America in general who didn't understand the country or the Vietnamese people. The sort who thought that all the Vietnamese needed was a firm hand to point them towards the democracy. The British guy is far more aware because of course the British by that point had lost most of their empire. And the fact the CIA guy doesn't understand the girl (who is I think working for the Viet Cong) is also a microcosm of America's problems in Southeast Asia.
The movie stars Michael Caine as the old British guy and Brendan Fraser as the CIA guy. Both give fine performances, so it's a really good movie. At some point I should get around to the book.