Right now the ebook versions of the Crucial Questions series by R.C. Sproul is free. The announcement on the Ligonier Ministries website reads:
To further help Christians know what they believe, why they believe it, how to live it, and how to share it, from today the eBook editions of R.C. Sproul’s Crucial Questions series will be free forever.So, there is no rush to go grab these. (Although I went immediately and grabbed all of them.) They have epub format available, but they also have links to the Kindle version and the iTunes version.
Check it out now and go get some questions answered.