Many ebooks from Untreed Reads are participating in the sale. Most of the coupons I've seen have been for 50% off -- I haven't seen any that are available free. They offer fantasy, science fiction, mystery and other genres.
https://www.smashwords.com/profile/view/untreedreads
One book I can recommend in particular is Waves in the Wind, by Wade McMahan. This is a very well written fantasy set in Ireland as the old gods are fading. The regular price is $5.99 -- it's available for $3.00 after the coupon code RAE50.
https://www.smashwords.com/books/view/542352
$3.00 USD