I think the 99c price point provides a lower level of risk, especially when the book is in the charts. You don't know whether or not you'll like the book. At $12.99 you may as well go to a bookshop not to buy the book but to try a few pages. At $2.99 you might be put off by the book's price because you don't know the author, but the risk is fairly low. At 99c the book is going to cost less than a newspaper.

If all books were 99c I'd definitely jump. At that point you're seriously disrupting print media to the point where future generations would refer to this period as a dark age.