Of course the Bible, but there are versions that are best for study and there are versions best suited just for reading and understanding.
I would also suggest the Jeffersonian Bible or "The Life and Morals of Jesus of Nazareth" by Thomas Jefferson.
I would hold off reading anything like "Letters from Earth" by Mark Twain until you have a good foundation in your faith. Let's just say that Mark Twain poses some interesting questions...