As of the writing of this post, you can pick up a free copy of the Kindle version of R.C. Sproul's Commentary on the Gospel of John. Just click the banner below to pick this up on Amazon.
Edit- I just realized the sales was Monday only. Here's a helpful note from Robert Warren - "During (most) Mondays (I don't know the specific time zone of reckoning; I check in the evening EST), you can search from the Kindle Store site at Amazon using "reformation trust" as the search term. When the results populate, change the Sort By to "Price: Low to High". If there's a freebie, it will appear at the top. There have been one or two Mondays in the last month or so when I wasn't able to find a freebie."