"Books" as a mass noun is a commodity, "books" as a plural noun is differentiated. (Somewhat by definition: anything that you think of as a mass noun is a commodity.) Amazon's goal was to get you to think of "books" as a mass noun, and among the general public, they largely succeeded. Book aficionados still think in terms of books as individual, differentiated titles, but these people are much more likely to go to small independent bookstores (where they will get individual, differentiated attention) than Amazon. The general public thinks "If I happen to need a book, I'll find it on Amazon".