I'm in agreement with the previous poster. The movie follows the book effectively for the first half of the film, but goes off the rails all at once they eliminate the Mexico run, eliminate the slow fall into near psychosis for Bucky, and do away with his slow estrangement with Kay that delivers a bigger payoff in the novel.
I read that de Palma cut nearly an hour from the film to get it down to a 2 hour runtime, and it shows. You can nearly pinpoint the spot where the scissors came out. It's a shame, really, because this film does have some moments. That said, having read the book, I was familiar with names, motives etc on the front end. I imagine this flick would drive a casual viewer downright crazy with all the hurried subplots and distractions.

I'd recommend skipping the movie. It's terrible, and it cut out a lot of what made the book work. For example, Bucky's trips down to Mexico and his harrowing escape from there with the phone booth and the woman hostage. Also, the case didn't seem to take anywhere near the toll on Bucky in the film as it did in the book.