If I had to rate this book, it would only get 3 of 5 stars, and that poor rating would all be because of the absolute horrid mother Bev is in both the movie and book, and how I can't get over that--even if she does go through some change to being a better mother by the end. I try to keep my ratings based more on the plot and writing, but I really find it impossible to bear that she is absolutely so horrible to her son at many points in the movie and book.
To compare the two, I'd had to say that I enjoyed the book more than the movie, even if the movie improved some of Bev's characteristics to be more likable.
In the movie, Bev can largely be seen as a product of one mistake--getting pregnant as a teenager--and all of her disgraceful actions after that are really a result of the situation she is stuck into because of it. She is still very clearly selfish, as highlighted by her son's responses to her as he gets older, but she is this way because of other people's actions more than because of her own.
In the book, Bev is much more experimental, and it isn't as much a snowball effect of her pregnancy that causes her and her son's problems, but her continuous bad choices that keep her stuck in the life she has began. While there are plenty of stories about young mothers not being able to come to terms with their children, I found Bev's total reluctance to care for her mother really off-putting; more so in the book than in the movie.
In the movie, we know from the beginning that Bev is selfish, and that her son resents her for it, but you can still see the actions that she takes to keep her son safe, and it is clear that she loves him. Though the book starts with her being upset that she cannot be with her son forever as he gets older and goes to college, this motherly affection is totally lacking in the rest of the book. In fact, for me, I didn't feel like she really transitioned at all.
There were a few major plot changes mostly based on what Bev did or didn't partake in, but the main change was Bev's situation and whether she caused each problem, or if it was a product of her early pregnancy. The way this was presented in the movie put Bev in a better light than in the book where she continuously pursues her own bad choices.
Overall, I didn't love the book or the movie, but I liked the book more than the movie. If you've seen the movie before, I don't personally recommend reading the book, though I have heard there are some that find it witty and hilarious.