I haven't seen the movie.
But now I want to.
I recently read the Fumiko Naito translated version of "Omoide no Marnie". Tough going at first, but rewarding in the end. For the first half of the book it's just about a lonely girl who has made a friend. And then suddenly her friend disappears in front of her eyes. Literally. What then happens is a heartwarming story that makes you want to re-read the beginning of the book again in a different light.
Language wise: no difficult vocabulary, relatively smooth to read for JLPT N2 and above.