Back in July, Tor Publishing started up an ebook club. Once a month during a 7 day window, they will give you a free download of one of their titles. If you haven’t joined, you can sign up by visiting here.
To give you an idea of the titles, here’s the backlist so far with links to their Amazon page for description:
July: The Three Body Problem by Cixin Liu
August: The Just City by Jo Walton
September: Gardens of the Moon by Steven Erikson