Then again, I don't think it's just some blonde from Harvard or an American girl.
The question was really about the relationship between
movie and book, rather than just "the movie" vs "the book"... right? 😉
Look... it's hard to pull out any statistical data that actually means anything here. The reality is that, according to Greenaway, a movie is basically just illustrated text. So, it makes sense that there are tons of adaptations of works that have already proven they work perfectly as a medium, which means there's a decent chance they'll work as a film too (take > for example). But on the flip side, you get this specific class of writers who only write screenplays because the format suits them, and they're already trained to think about how a script will translate to the screen (I'm immediately thinking of Allan Ball and his >).
When you're dealing with a book, though, you have to consider the form itself. Like, short stories make for the best foundation for a film because they provide those core plot elements. Everything else gets filled in based on the inspiration and vision of the director, along with the rest of the creative crew—let's say, the production designer, the costume designer, the composer. And sure, the producer usually has something to say, too. Novels and novellas are a bit more expansive, so you need to perform a kind of reductive adaptation. That’s serious work; you have to distill what’s essential to understanding the literary work into the constraints of film as a medium. A movie's success rests almost entirely on the ability—though not exclusively—of the screenwriter to adapt the story down to the "bare bones," much like a short story does. Of course, a novel or novella provides so much extra data and info that the creative team's job ends up being a bit easier.
It would be interesting to try and pick a successful literary work and see if it's already been adapted within this last century of film. Maybe a few times... maybe not at all.