The Lost Entwife is hosting a mini-challenge on what book we'd like to see made into a movie and then cast a character. THe first one that springs to mind is Her Fearful Symmetry by Audrey Niffenegger. I just finished it this week and really enjoyed it. I think it would make a good, although creepy, movie.
I think Emily Blunt would work really well as the main characters, twins Julia and Valentina. She's dainty enough to play them and is a strong enough actress to play the dual role, including playing the two very different personalities.