From Digital Inspiration: This chart compares the size of all the popular online ebook stores from Amazon (Kindle), Barnes and Noble (Nook), Apple iBooks and Sony. (2010)
It is interesting to see the relative size of online bookstores compared side by side.
Informative. 8/10
Full post: http://www.labnol.org/internet/online-ebook-stores-compare/17507/