The sixth Harry Potter book will hit shelves this fall, and doubtless it will sell millions of copies. But it’s been seven years since Harry first burst upon the scene, and the legions of devoted young readers that made him such a phenomenon are seven years older as well – many of them heading off the college, in fact. So how do you keep your newly adult audience interested in what is, after all, a children’s book series? You can age the hero, of course, but subtly increasing the complexity of the storyline will help, too. And you can always count on good old-fashioned reader loyalty…