Once upon a time it was all free.
Only a couple of weeks ago Scribd was just the most popular free online document sharing service; today it's also an online store that will sell your educational texts.

From their site, "In the Scribd Store you can buy exclusive books, research reports, never-before-released chapters of Lonely Planet city travel guides...for less."

What's the deal? Scribd will take 20% of each sale and give the rest to the copyright owner.
According to eSchool News, Scribd's advantage is that their documents will be able to be read on any type of device. The New York Times says they are turning themselves into a kind of "vanity publisher."

From Scribd's website About Us, "Scribd’s vision is to liberate the written word — to turn everyone into a publisher and create the best possible reading experience on the web and mobile platforms."