No 1 ~ AMAZON
Despite the seeming victory of Macmillan in its battle to force Amazon to accept the new “agency model” publishers have a sensible fear of Amazon. Like all businesses that sell their goods to consumers through intermediaries, publishers are forced to subject themselves and their products to the requests and “suggestions” of the retailer.
Amazon controls a large portion of the online consumer connection to books. They may not be the best at this, but they are surely the biggest. They have been on top of pretty much every trend in publishing for some time: