Let's set the market price at 100 (can be any number, but 100 calculates easier).

He sells for 80. Of every 12 books he sells 11, making him 880.

His profit is 10%, so the cost price for the 12 books is 800.

The cost price per book is then 66.67.

The market price profit margin is then 100/66.67 = 1.5 -> 50% above cost price.