If you live in USA or Canada, Tor.com have a great offer starting up - a free ebook every month if you sign up to their mailing list.
See
http://ebookclub.tor.com/ for full details.
The first book on offer is
The Three-Body Problem by Cixin Liu.
[Update: The first month's offer is now over. But you can still sign up to their mailing list to get next month's free ebook, although we won't know what that is until next month]