Agency agreements
Neither Apple nor Amazon as well as the five publishers, namely Simon & Schuster, HarperCollins Publishers, Hachette Book Group, Penguin Group USA and Macmillan commented on the investigation.
Some experts claim the agency model has pushed priced of e-books higher, especially as publishers set the prices which are identical for different websites.
Random House is reportedly the only major publisher to reject the agency pricing model, sticking to an old fashioned wholesale set-up for its digital books. This allows the retailers to set their own prices and gives them the opportunity to undercut their competitors.
A Random House exec is previously believed to have acknowledged the economic advantages of using a wholesale model, where it receives half the book's cover price irrespective to what the seller charges.
However, other publishers have said agency pricing allows different booksellers to make a profit from selling books, which could otherwise prove unlikely if a couple of the giants were systematically undercutting other retailers, therefore arguing the model encourages competition.
What is certain is e-book sales are on the rise, boosted by a new generation of e-readers such as Amazon's new Kindle and the arrival of tablets with e-reader capabilities.
According to the Association of American Publishers, e-book sales at 13 publishers have risen 163 percent over a year to over $29m in May. Amazon itself has also recently passed a milestone as Kindle books now outsell hardbacks.
