I'm going to surmise that this is one of those rare occasions where the movie actually exceeds the book.
Patricia Highsmith was amazing, of that there is no doubt. However, this book was extremely frustrating to read because there are so many terrible decisions being made by the main character, Guy Haines, whose encounter with a psychotic murderer is a terrible turning point in his life.
The plot is quite different from what I thought I understood it to be - and perhaps the movie aligns more with my misunderstanding. I went into it thinking it was more of an inverted mystery, and was interested to see where the mistake was made for the investigators to figure it out. I wasn't expecting one half of the plot to be a reluctant participant, and nearly as much of a victim as the murder victims.
I think that the biggest problem with this book is that it felt about 100 pages too long, and took fairly close to forever to get to the point. While Highsmith excels at building suspense, the pacing was way off in this one. In addition, the end was sort of anticlimactic.
I'm still a card-carrying member of the Patricia Highsmith fan club, but this was a bit of a disappointment.