To download a book, search for a title over at Google Books. Public domain titles will have a download link in the upper right corner. Which brings us to the first major difference between the Kindle and this Google-Sony open book strategy: while Amazon only offers 300,000 titles, Google's million books aren't, for the most part, the most attractive ones, and Sony's own ebook library doesn't offer a choice as good as Amazon - at least when it comes to modern titles.
Sure, if you're interested in an oldie, such as the Memoirs of Granville Sharp, Google's library is a good choice, but if you're looking to buy a digital copy of the latest bestseller, you're more likely to find it on the Kindle than in Google's library and Sony's ebook store combined. You can sometimes buy an ebook online and then transfer it to your Sony ebook reader, but on the Kindle it's simpler and easier to do.