The Spiderwick Chronicles

This movie is based on 5 ultra-short children's novels. The general idea is the same, but there are major differences. The children's parents' divorce is barely mentioned in the books, but becomes a major point in the movie. Mulgarath is mentioned immediately, where the books do not mention him until book 5. An entire book (#4, The Ironwood Tree) is left out completely, except for a short scene where Mulgarath poses as the childrens' father (played by Andrew McCarthy, wow)

Jordan/Simon, played by the same actor, does a great job. I thought it was two different kids playing the role.

The visit to Aunt Lucinda is Mallory and Jared only, and they have to go through an underground tunnel. A magical circle around the house is extremely important in the movie, but does not exist in the book. The griffin was Spiderwick's pet in the movie, whereas in the book the children nurse him back to health and he is their pet, not Arthur's.

Arthur is held captive by sylphs in the movie; in the book, it is elves. In the movie, Lucy, his daughter, comes with him in the end, and magically changes from 86 to 6. In the book, Arthur steps out of the glad and instantly disintegrates. In the movie, the children fight the evil forces with tomato sauce and knives, inside their own house. In the book, they destroy Mulgarath only and the others lose their evil intentions.

There are several other key differences. But it still results in a good movie, maybe a better movie than if the books were followed exactly. And the general spirit of the books is retained.