Scrib.com uses the logic of tagging and recommending--"2 of your friends liked..." etc. If I like a book, I can "readcast" it, or announce its presence and goodness to all of my friends on my social media tools.
Thus, published and self-published books and documents of all types hang out side-by-side, waiting to be recommended.
Many of the books are available in PDF's embedded on the site itself--you simply scroll down. I downloaded one title for free, was asked to pay for others, and can't figure out the pattern.
Scribd 101 was somewhat helpful in this regard, if not answering that particular question. The sprawling, benignly chaotic nature of the web site necessitates a bit of an orientation.