It took e-books less than four years to achieve world domination—or, at least, Amazon.com domination. Amazon now officially sells more electronic books than printed books. Since April 1, it has sold 105 e-books for every 100 printed books. What's behind those numbers? Printed books include both hardcover and paperback versions, as well as printed books for which there is no electronic version; the comparison excludes free e-books. (More Amazon stories.)