Here's an example of how to get rich by piggybacking onto something that is already successful: Ulysses Press, a California publisher, says they have 335,000 copies in print of "What Will Happen in Harry Potter 7?" (Let's hope they've sold most of those by now, because in a couple of weeks it won't be a very relevant title...).
The book was written by two young guys (one 17, the other 20), who created the Potter fansite, MuggleNet.com. The older one was originally turned down by an Ivy League university, but appealed that decision by sending them proof of the success of this business venture. They took him.
Here's how the site describes the book: "As anticipation of the final Harry Potter book intensifies, a debate is raging among fans about what's in store for Harry and the rest of the gang at Hogwarts. In this book, the experts at MuggleNet.com present a wide range of hard facts and bold predictions about the most popular storylines, favorite characters, and final outcome of the Harry Potter saga. Drawing on their intimate knowledge of the previous six books, as well as tips and suggestions made by millions of MuggleNet.com fans (not to mention a personal interview with J.K. Rowling), the authors offer answers to the burning questions of Harry Potter readers everywhere: Will Hogwarts School be open for Harry's final year and will Harry even be in attendance? Will Harry's quest for the remaining Horcruxes be rewarded? Where do Severus Snape's true loyalties lie? And, most importantly, will Harry survive the final battle with Lord Voldemort?"
By the way, the very busy and professional site looks like the authors have had a lot of cooperation from the publisher and from Warner Brothers--of course, anything that fuels interest and participation will be good for Harry. By now, most publishers and film companies have learned that it's far better to cooperate with fan sites and publications than to fight them.