Fifty Shades of Gray. Harry Potter and the Philosopher's Stone. Wool. These are all examples of the best way to guarantee multiple sales of your books: writing a series and creating a sales funnel. JK Rowling may have had Harry & Co. all planned out years in advance, but Hugh Howey and E. L. James thought they were writing one-offs. Once sales exploded on their first books, they were smart enough to grab the idea and expand it into multiple volumes, creating a larger body of work and multiplying their fan bases enormously.