From Sony’s Site, "Together with Google, Sony brings you access to over 500,000 public domain books for free. Now you can search, browse, and read over half a million public domain books preserved by the world’s great libraries on your Sony Reader."
Amazon link to buy PRS-505:
Sony PRS-505 Portable Digital e-Reader System (Silver)
PRS-500 owners can trade in for the new PRS-505 and receive $100 credit, details here: