The main difference is the tone. The novel is a lot darker, and the ending is more bittersweet. In fact, the entire 3rd act is different. In the novel, Pat is an unreliable narrator (it's clear that the reality he has built around himself is not the entire truth), and, we don't find out about what his ex-wife did until the end. His relationship with Tiffany isn't the main storyline. She's as much of a supporting character as Pat's family, friends and therapist. Also, she's very quiet. The two of them don't talk much to each other in the book, whereas in the movie they talk a lot. Another detail is that, in the movie, he's diagnosed as bipolar whereas, in the book, he isn't explicitly diagnosed as anything.
Originally the narrator is called Pat Peoples, unlike at the movie which happens to be Pat Solitano. Pat's trigger song in the book is anything by Kenny G, especially "Songbird", as well as any other jazz music. In the movie it is "My Cherie Amour" by Stevie Wonder. In the book, the dad doesn't want to spend time with Pat, but in the movie Pat is reluctant to spend time with his dad. In the book, the dad doesn't talk to Pat, whereas in the movie he does. In the book, the parents have marriage problems, the mother cries much more in the book and she comes home drunk at least twice in the book but never in the movie. In the book, Pat hears and sees Kenny G in his nightmares. Kenny G isn't mentioned in the movie at all.
In the book, Pat is given a Hank Baskett jersey by his brother, and his brother and friends subsequently call him Hank Baskett. Baskett was an undrafted rookie. He became the first Eagle player to catch 2 TD passes longer than 80 yards in the same season. In the novel, Baskett is symbolic for Pat. Both Baskett and Pat have to go through a long evolution. In the movie it changes to a DeSean Jackson jersey. In the book, Pat takes much time to actually mention sensitive stuff he thinks about, but in the movie is very outspoken and speaks his mind uncontrollably. The dance "competition" is revealed to be a showcase of dancing involving primarily high school kids rather than a scored contest.