The digital version of the book was on sale for $2 about a week before the movie. I bought it then but haven't started on it yet. It's getting close to the top of the list though. The book started out self-published (I assume on Amazon) and I recently read a series that started out the same way (self-published then got hugely popular and was picked up by a major publication and printed, only the series wasn't turned into a movie). It was called the Wool Series. I really liked the first book of that series but by the end of the third (and last) book I was just reading to finish the series.