I loved Rose Red the movie and unfortunately, hated The Diary of Ellen Rimbauer the movie. I had hoped the book would be better. It wasn't.
The whole point of The Diary of Ellen Rimbauer was Ellen being obsessed with John, even when everyone told her he was bad for her. She didn't care. So she married him. Big mistake.
The rest of the story, as far as I bothered reading it, was Ellen complaining about John, his philandering and what he put her through. A terrible prequel to Rose Red, which is a fantastic story. So I suggest, if you watched or read Rose Red and loved it and thought you'd continue the story with The Diary of Ellen Rimbauer for background reading...don't!