I had read the book and just now saw the movie on Pluto. The book was a lot better. They cut a lot of things out for the movie and that just kind of ruined it. Although it was interesting to see the characters from the book come to life, there were too many differences.
In the book:
Nell has a Great Aunt, Mac's sister, who doesn't appear at all in the movie. Her aunt is the one who introduces her to Bonnie instead of Bonnie randomly walking up to her.
Lisa Ryan didn't go with Nell to Winifred's apartment and Nell didn't send her to visit Bonnie. Mac sent his secretary who also doesn't appear in the movie.
Jed Kaplan never stalked Nell and he was not killed.
The part that bothered me the most is that the movie leaves you hanging at the end with regards to the election, something that was revealed in the book.
I could go on and on. I can understand that some things from the book weren't necessary but they changed too much of the story.