I don't really think there's any twist to this story. But from what I've read, it does sound like Night actually attempted to give the film an ending of sorts while the book just kind of winds down without any real conclusion.
The book's ending (from a synopsis I read):
From what I've read...
I've read the synopsis of the book, and I think I'll skip this movie. Unless Shyamalan has re-written or added to the ending, it seems like this is something that would just leave me annoyed when the credits roll.