Crossway is selling all of their eBooks for $5.99. Some of the eBooks equate to better savings than others.
Among the best deals to be had in this sale is Wellum and Gentry's book Kingdom Through Covenant. I have the book on Kindle, and it makes a good transition to the device. I like having such a large, heavy book in the palm of my hand. Theologically, while there is much to benefit from in this book, there are other serious problems with it. The fact is, if you want the book, the hardcover cannot be had for less than $30, and even the eBook version is normally quite pricey (they had a hard time making it and getting the Hebrew characters to appear on the Kindle). If you buy it, you'll be getting a good deal.
As someone who is preparing to enter the Gospel ministry, I have been planning for some time to read Paul Tripp's book Dangerous Calling. Now, we have almost no excuse, since this book, which normally sells for $20 can be purchased for $5.99. The hardcover is on sale for 50% off at Westminster Books, as well.
Kevin DeYoung's book The Hole in Our Holiness can be bought for the Kindle for $5.99. For those who want a physical copy, Westminster Books is having their own sale, offering the same book at 50% off.
Finally, I will mention a book that is turning out to be one of my favorites of last year. I'm almost finished reading it and will be giving a glowing review in the next week or two. It is Rob Lister's PhD dissertation, entitled God Is Impassible and Impassioned. $5.99 is a steal for a book this important. I recommend it to all. The paperback copy is also available at Westminster Books.