Some books are newly free for the Kindle* (or the free Kindle apps):
The Holiness of God by R. C. Sproul — His book rarely stay free long, and this is one every believer should read. Get it while it's free!
The Strategy of Satan by Warren Wiersbe — It's a preorder; I know nothing else about this book, but the description says it looks as Satan's attacks and defeating them by obeying God's truth.
10 People Every Christian Should Know by Warren Wiersbe — A shortened ,but free, version of his "50 People ...".
*On a related note, I got a Kindle Fire, and I have to say it's really neat — and about the cheapest tablet you're going to find.