You can download free books from this very site, at the top of the page click "e-books" and you can download lrf or epub books for your reader.
other places to try are
www.kobobooks.com,
www.booksonboard.com, inkmesh.com, gutenburg, google books, that's all I can think of off the top of my head, there are a lot more sites out there... You can also read library books on your sony 600, make sure to check if libraries in your area offer ebooks. Is there any particular genre / author that you are looking to find?