I am not a big R.C. Sproul fan since I am not Calvinist nor Amil, but I think there are some good ones in here... and they are free!
http://www.amazon.co...uestions kindle
Some of the Titles
Who is Jesus?
Can I Trust the Bible?
Who is the Holy Spirit?
Does Prayer Change Things?
What is Baptism?
What Does it Mean to be Born Again?
ect...