Oh. Well, legally distributed ebooks are generally free for one of the following reasons:
1) texts out of copyright, such as Austen, Bronte, Shakespeare, etc.
2) promotional freebies. B&N, Amazon, Kobo, etc., all are bookstores which also release freebies from time to time.
3) books released free specifically under other conditions (including creative commons).
Some of the sites linked to, such as Gutenberg and Mobileread, stock mainly ebooks out of copyright. Also, you have a Sony, so you can always borrow books from the library if they have a collection.
If you're interested in the Star Wars books -- BTW, Karen Traviss has written some good Star Wars books, worth paying the $3-ish IMHO -- maybe you like sci-fi?
Baen is a publisher that offers some good sci-fi ebooks for free. Good way to find new reads.