MTV has an interview with New Moon screenwriter Melissa Rosenberg in which she discusses what changes she made to the novel and why. This will probably be of interest to fans of the book and those interested in the craft of film adaptation. Check it out here.
That said, in editing the source material, Rosenberg didn't (and probably couldn't, for fear of angering the Twilight fan base) fix the major problem of the book: that the entire second act of the story bears no relevence on the third act. [SPOILER ALERT] The film could quite literally have skipped from Edward leaving (which concludes the first act of the story) to Bella cliff jumping and her journey to Europe, and removed the entire Jacob storyline. [END SPOILER ALERT] I know Twilight fans will likely point out that wolf-boy and his friends become an important part of the following stories, and while that may be true, it is also nevertheless important for this film to stand on its own and deliver a coherent story.