As I'm sure just about everyone has heard by now, J.K. Rowling has written a book for adults. Of course, her previous books, a series about a boy named Harry Potter (of which you may have heard) were read pretty extensively by millions of adults worldwide. So what makes this new book "for adults"?
The answer is that there is no answer, at least not yet. In the two weeks since the book was announced, all that has been revealed is the publisher (Little, Brown) and that yes, the book is in fact for adults. When previous authors of children or teen books have made the jump (or perhaps small step) to adult books, the differences have been clear, if for no other reason, because the author emphasized them.
In 2008, in the midst of Twilight series madness, the author Stephenie Meyer published The Host. The Host was also published by Little, Brown and to much less attention than any of the Twilight books, while still debuting at number one on The New York Times bestseller list and being optioned for a feature film. The main thing that sets the book apart from her bestselling teen series in terms of age demographic is the age of its heroine. The target audience seems to be the same as her teen books—older teens and younger grownups, primarily females interested in romances. Neither Meyer nor reviewers emphasized much distinction besides reiterating the nominal one.
Eoin Colfer, author of the bestselling Artemis Fowl books, took an opposite approach when publishing a book for adults, Plugged, published by Overlook. From the tagline—“If you liked Artemis Fowl…it’s time to grow up”—to the content, everything about the book announces that this is an adult book. Colfer broadcasts that Plugged isn't for the same age demographic as his middle grade series by highlighting the cursing, sex, violence, and drugs in the book.
Both Colfer and Rowling left their children-focused publishers for their adult books, and both originally write for a younger demographic than Meyer. However, it is hard to imagine the far less sarcastic Rowling writing a book about a balding bouncer in New Jersey. Without even a genre announced, however, anything is possible.