Harry is right. Since I've bought the Cybook, I've spent hours browsing the free ebooks catalogs, in wonder at the many titles available, to discover or re-discover ad lib... Just have a look at
Feedbooks, for instance. The books labelled as formatted for Sony PRS-500 work fine on Cybook, because it's the same screen size. See also the
Free ebooks section of Mobipocket, for classical books in PRC format...
And thanks to libraries who scan and digitize their out-of-print books, you can find a lot of material that hasn't been available commercially for decades. My father, who loves ancient literature, got very interested about ebooks when he learned that a 16th Century translation of Plutarch was available as a digital text, courtesy of the University of Ontario, but was out of print since more than 100 years!
And then, of course, if you like to read SF, there's
a growing number of authors who follow the Cory Doctorow path and offer their ebooks for free under Creative Commons licenses. In addition to Cory, you have Charles Stross, whose novel
Accelerando was packaged with the first Cybooks, then there's Rudy Rucker, Peter Watts, James Patrick Kelly, and more... Just now,
Joshua Klein added himself to the list!
Don't forget also Baen Books, for lots of science-fiction, fantasy and alternate history novels sold without DRMs. As Baen/NAEB partnered with Bookeen to develop their own brand of ebook reader, I guess they don't have any intention to tye themselves up with Amazon...