While in some ways Harry Potter qualifies in my mind as a transmedia property, it would be tough to argue that the saga is an actual transmedia narrative. This is for the very reason that you mention — Rowling has placed some pretty high fences around the canon as it exists between the covers of her books. The films, videogames, toys, audios and user-generated content in no way add to that canon. Most merely repeat or repurpose the tale. Interestingly, the same can be said thus far of Meyer’s Twilight narrative, though I’ll probably be slammed from both sides for mentioning the two properties in the same paragraph! Ha! I’m actually looking forward to a book-born mass media phenom that posits itself as a true transmedia storyline. Coming soon…