According to their email:
BooksOnBoard will be discounting ALL discount eligible eBooks 35% Plus 8% Back in Rewards for a net 43% OFF SALE good ONLY till Midnight (CST) tomorrow, September 20, 2012. Seventy percent of the eBooks on our site are eligible for this discount as well as all audio books. Don't miss out, this is a tonight and tomorrow only price promotion! No promo codes required, no limit on titles.
Edit: I'm quite impressed. I purchased 4 books which would cost around $30 on Amazon. The price on Books on Board totaled $18.50. Since my order was over $18, $2 was reduced from the price leaving a princely balance of $16.50 to be paid

Plus I now have a credit of $2.24 for future purchases.