But the point is that most people will take the literal definition of "book". They wouldn't differentiate between hardcover, paperback, ebooks, audiobooks. If an "expert" said "90% of books sell less than 99 copies", without any context, most people will assume that the book sold less than 99 copies across all its manifestations and in a lifetime. The article puts light on the fact that there's more to that statement than what it means in a first glance.