The movie opened on September 30. Keith R.A. DeCandido wrote Serenity, the novelization of the movie.
So, why read the novel version of a movie? Especially since I also bought the screenplay? For the same reason I read books-upon-which-movies-are-based. I want more; I want to find out what characters are thinking, maybe get multiple viewpoints on the same event. I want to read scenes that weren't in a movie, because it was only two hours long. I want background. And while I know how the story will end, I also know that a book will tell a story a bit differently from a movie. I want to read that different version.
Admittedly, many novelizations fail to deliver and are poorly written.
But some novelizations are great. Serenity does deliver; DeCandido's novel is a real treat. He obviously cares about his finished product, and the book contains flashbacks, additional scenes, insights into characters. As with the film the focus is Mal and River; but there are also glimpses into other characters. Here's a tease: we get a bit about Wash, flightschool, and Mr. Universe.
Other novelizations that deliver: the original Battlestar Galactica novel tie-ins were some of the best I have ever read. (OK, so I was ten at the time...) No, seriously: they included scenes and backstory that fleshed out the story and created moments that I still remember.
If you're interested, To Some, a Movie Is Just an Outline for a Book is a fascinating look at the book based on movie business.