Deduced from my somewhat limited study of people I've met in airports and at Belt events, about 82 percent of Americans think they have a good idea for a book.
Now, it's definitely a very feel-good and motivational thing to say everyone has a book in them, but I do not believe this. (I also do not think everyone has a movie in them, or a restaurant, or a surgery, or whatever other activity requiring expertise people assume they could totally do). I do think everyone has a good essay in them, though. Belt's city anthology series, books containing dozens of essays about a city, often written by people who have never published before, bears this out to a certain extent.