When Marnie Was There, the 1967 novel by Joan G. Robinson. It was #18 on Hayao Miyazaki's list of recommended children's books, so this shouldn't be a shock, although it's worth pointing out that Marnie will be directed by The Secret World of Arriety's Hiromasa Yonebayashi, as Miyazaki still maintains he's retired.
If you read this summary of When Marnie Was There, courtesy of Amazon, it sounds like it was written with a Ghibli movie in mind all along:
Anna hasn't a friend in the world - until she meets Marnie among the sand dunes. But Marnie isn't all she seems...An atmospheric ghost story with truths to tell about friendship, families and loneliness. Anna lives with foster parents, a misfit with no friends, always on the outside of things. Then she is sent to Norfolk to stay with old Mr and Mrs Pegg, where she runs wild on the sand dunes and around the water. There is a house, the Marsh House, which she feels she recognises - and she soon meets a strange little girl called Marnie, who becomes Anna's first ever friend. Then one day, Marnie vanishes. A new family, the Lindsays, move into the Marsh House. Having learnt so much from Marnie about friendship, Anna makes firm friends with the Lindsays - and learns some strange truths about Marnie, who was not all she seemed...
When Marnie Was There is due to premiere in Japan next summer.
[Via Anime News Network]