Smashwords has thousands of books in its catalogue for "Read an E-book Week"-- prices anywhere between 25% off to Free. Fiction, non-fiction, you name it.
Coupon codes to be used at check out: "RE100" 100% off /"REW75" 75% off/ "REW50" 50% off /"REW25" 25% off
http://www.smashwords.com/books/category/1/newest/1
While you're there, you might take a look at the page for Goblin Moon (now selling at 50% off)