Again, daily unit sales are estimated by sales ranking, using publicly shared data from dozens of authors who have logged the correlation between rank and daily purchases (included among those authors are the two involved in this study). 5 Some obvious things immediately jump out. The first is that Amazon has an incredible ability to market their own works, which shouldn’t be too surprising, considering it’s their storefront. We see from this and the previous chart that their 4% of titles command an amazing 15% of the sales. That’s impressive. It’s nearly 4 times the average unit sales volume per book. Now look at the Big Five, who with all their marketing efforts and brand recognition actually end up with pretty average per-book sales: a mere times the overall average.