Where the Red Fern Grows
Before I get into the details of my comparison I want to be clear: I watched the original 1974 film adaptation, not Disney's 2003 remake. If you've seen the Disney version I'd love to know how it compares. Feel free to share your opinion in the comments.
For the most part, this is a pretty faithful adaptation of the book. While small details frequently deviate from the book this is often the case with movies. The spirit of the story remains the same, with one significant exception. The big coon hunt is a much shorter scene in the film than the book and has a vastly different ending, giving Billy more of a moral victory and imparting a life lesson rather than the literal victory in Rawls' original story. It's a slow moving story, both on the page and on the screen, but the movie lacks much of Billy's internal journey. Without the monologues on sportsmanship, the relationship between a boy and his dog, and the nature of God, there is surprisingly little to this story. It plays well on screen, though, especially if one can handle the rather cheesy soundtrack.