Half price books sitewide at Mills & Boon (including existing discounts)
On the plus side though the discount also appear to work on their existing special offers.
I tried applying this to their "Sizzling Collection" special offer which was already marked down to $7. This includes 4 ebooks ("The Spy Who Saved Christmas" by Dana Marton, "A Cop In Her Stocking" by Ann Voss Peterson, "Christmas Countdown" by Jan Hambright, and "Silent Night Stakeout" by Kerry Connor) and after applying the coupon it comes down to just $4.25.
There are another 6 special offers available ranging between $7 and $15. In terms of their overall selection, I can't tell exactly how many books there are under $5.99 (as they don't appear to have any kind of sort function on their website, annoyingly) but there look to be many of them under each category.
So definitely worth having a browse if you're a Mills & Boon fan as you're likely to find one or more books/ebooks for just $3.99 a pop.