Crossway has good deals on several books now through April 20 (Kindle editions).
Preaching the Cross is a good collection of written versions of the messages delivered at the 2006 (inaugural) Together for the Gospel conference. Included are two very helpful messages by Drs. Ligon Duncan (Preaching the Old Testament) and John Piper (Why Preaching Is Particularly Glorifying to God), as well as sermons by the other speakers.
The Gospel and Personal Evangelism (which I have reviewed here) is an excellent work and only 99 cents this week.