What? I'd be shocked if Amazon even published a majority of books sold. There is no way they are monopoly or even close. I looked and couldn't find reliable stats on it. Does anyone else have any?
A majority by number of unique titles, or by sales volume? I would not be surprised to see Amazon publishing a very large fraction of 'long-tail' (almost self-published) books, and these likely make up a large fraction of all titles published.