Parents reading books to kids, students reading books for classes, and people who end up reading at least one book a year for work (many teachers or professors, for instance) set a fairly-high lower bound on this.

Much of the rest is people who exclusively read very easy books from one or two genres (“romance”, true crime, airport thriller/mystery, young-adult fantasy, and self-help/business-guru, mostly). That’s especially going to dominate the shelves of the set of folks with books-read counts far higher than one per year. Whether that crowd counts much toward a measure of the exercise of quality, general literacy, is a judgement call, but those readers are the engine of what little remains of the market for new books.

(There’s a niche market that’s commercially viable that involves books laser-focused at being optioned for TV or movies, but it’s as cliquish as you’d expect and hard to break into, and of course other genres still support a tiny number of super-stars)