The Cathy's Book approach: Gimmick?
Hugo Cabret: Hook?
(If my assumptions are correct, what's really the difference between using graphics in a novel and using interactivity in a novel, then? Neither has much to do with the core story. Is it just a matter of the value judgments concealed in hook vs. gimmick?)
Books written in IM: hook or gimmick?
Scratch n' sniff ERs: gimmick? or hook?
Googlie eyes, die cuts, etc: gimmick?
(I'd say so, but those googlie eye books sell enough to perhaps call that a hook?)
Urban vampire books: started out as hook, has turned into gimmick?
Ok, we already seem to have forgotten that effective gimmicks are hooks (though not all hooks are gimmicks). Please refer to the venn diagram. And if you'll refer to my definitions of "hook" and "gimmick", I think you'll see that there are no value judgements involved.
Cathy's Book: The text itself wasn't bad, but the extras, while fun, were an effective gimmick. (Which, once again, means they were also a hook.)
Hugo Cabret: The extraordinary visual storytelling is not a distraction from what the book is. It is part and parcel of what the book is. That means it is not a gimmick.
Vampire books: ditto. Either you want a vampire story or you don't. You aren't going to think "Ooo, vampires!" in the store and then get the book home and think, "Wait, there are vampires in this book! I didn't want a book with vampires!"
Books written in IM: Arguable. Some would argue that it's no different than traditional epistolary novels, which would make it no kind of hook (gimmick or otherwise). Others would argue that anything that makes a book unreadable does distract from what it is, so perhaps it could be a gimmick. Ok, now I'm just being mean.
Scratch and Sniff: gimmick.
Googly eyes: gimmick.
Die cuts: varies from book to book. The die cuts in First the Egg were not a gimmick. They were absolutely integral to the book. The die cuts in Penis Pokey... well, I'm hoping they're a gimmick. I guess it depends whether people are actually using them.
Ok, here's a fantastic example of a gimmick: the squishy, stretchable (surprisingly realistic-looking) plastic tongue in the cover of Chewy, Gooey, Rumble, Plop. Kids who are in no way interested in learning about the digestive system are going to beg for this book just for the tongue. They'll get it home and play with the toy parts and not read a word of it, and the parent who paid hard-earned money for it will be irritated. That's a gimmick. If you had a product that was solely a squishy, stretchable plastic tongue, then its appeal would not be a gimmick, because it wouldn't be distracting from anything else. Are we clear now?