There are loads of free ebooks available for the Sony ereader. The links above get you started on finding free classics. I see in another post that you have already discovered the free google books on the Sony ebookstore.
You should also check if your local library has ebooks. Here is a link to the San Diego Public Library website as an example.
http://sdpl.lib.overdrive.com/242E32...en/Default.htm
Also, many publishers and authors have free promos. You can find them on the Sony ebookstore in the category "Bargain-Priced". However, sometimes they don't have them all listed. I use the website inkmesh to find what is free. Then I do a search on the Sony Store.
http://www.inkmesh.com/free-ebooks/?site=sony
Have fun looking for those bargains!