The book by Tim Tharp and the 2013 Hollywood adaptation of this movie differed in a few big ways, and mostly that different lied in their endings.
The book and movie both tell the story of Sutter, the boy everyone wants to be friends with and is the life of the party, and all the ways in which he is a going to remain just that way. Sutter is a drunk who isn't going anywhere, and he unexpectedly falls for Aimee, and begins to be a bad influence on her life. They're happy, they're together, but Sutter isn't the right choice for her.
The biggest difference between the two versions comes in their endings. Spoiler alert!!! In the book, Sutter tells Aimee to go without him to St. Louis, and shows no resolution to follow her. In the movie, he also tries to let her go, but ultimately goes after her, and the movie ends on a hopeful note.
I really don't know how I feel about the ending: overall, the book is more realistic in the ways it shows their relationship and the problems it has, while the movie is more romanticized, almost to the point of Romeo and Juliet. This didn't really bother me much, but the change of ending has left me confused. In some ways, I feel like the ending of the movie is more realistic: isn't it more likely that Sutter will remain selfish and want to stay with Aimee, even if it's not best for her. Do we really believe, like in the book, that Sutter will let her go--which is ultimately the better choice for her? I'm not convinced.
Either way, I really enjoyed both the book and the movie, and maybe even liked the movie a little bit more than the book! I definitely recommend both of them.