OmniLit have a 50% rebate offer on eligible ebooks (includes HarperCollins ebooks) for today 31 October only. (US Central Time)
You can use the rebate to buy any ebook (not just the rebate-eligible ones), but you don't get another rebate on the rebate (i.e. you only get a rebate when spending real money). This means that the 50% rebate is really equivalent to a 33% discount.
But the books eligible for the rebate also count towards the buy-10-get-1-free scheme, which is worth at least 9% (or more depending on which book you choose as your free book), so in total I figure the 50% rebate and buy-10-get-1-free points count as equivalent to a discount of at least 37.5%
(For example say you bought 20 rebate eligible books, all at the same price. Then the 50% rebate would be enough to buy another 10 books, and the buy-10-get-1-free points would get you another 2 free books. So 32 books for the price of 20, a 37.5% discount.)