Amazon created the system 10 years ago. Neither Amazon nor Barnes & Noble will divulge the algorithms they use for rankings. What Amazon will say is that sales rankings are updated hourly for millions of items. The sales history counts, but recent sales are weighted more heavily.
For most books, it does not take many orders to increase rankings. Knowing this, authors, publishers, even nonprofit organizations like MoveOn.org will send out e-mail blasts asking people to buy a book at a set time, or buy up copies themselves. Some authors get their friends to write reviews or even write a positive review for a rival book and mention their own title.
Breaking into Amazon’s top echelons and the “also bought” recommendations can have a viral effect. “If you crack the top 50, even if it’s only for an hour, you’re significantly higher in all categories,” said Mr. Kirchner of Ingram Publisher Services. “You could probably go online at 4 a.m. and make any book you wanted a best seller for an hour.” Article