From the page to the silverscreen

Nim’s Island Wendy Orr
The film stays true to the plotline of Orr’s 1999 children’s book about a girl named Nim (Abigail Breslin) who lives with her father on a remote island, except for a few scene omissions (e.g., Nim’s building of a coconut raft). And the film fleshes out Nim’s pen pal, author Alex Rover (played by Jodie Foster), from a mere recluse in the book into a hallucinating, full-blown agoraphobe.
Fidelity-o-meter: 4 out of 5

21: Bringing Down the House Ben Mezrich
There’s already been a public fuss over 21‘s WASPification of the mostly ethnic cast in Mezrich’s 2002 book (originally titled Bringing Down the House), loosely based on the true story of an MIT nerd squad that beat Vegas at its own game. The movie takes even greater liberties with the plot: Chase scenes, comic sidekicks, and tear-jerking backstories are inserted into the already stranger-than-fiction tale.
Fidelity-o-meter: 3 out of 5

The Ruins Scott Smith
Adapting his own 2006 horror novel, Smith adds plenty of haunted-house frights to what was once a slow-burn thriller about college-age tourists who stumble onto a leafy deathtrap deep in the Mexican wilderness. The movie keeps most of the book’s plot points, though it sometimes swaps the characters acting them out. And a new, radically different ending leaves room for a second pairing of Hollywood and vine.
Fidelity-o-meter: 2 out of 5