Obviously the best book to read here is the Bible but here are some other suggestions to help you to work out what you believe and why:
Why Jesus? - Nicky Gumbel (this little booklet is a good starting point and is great to give to your non-Christian friends)
The Case for Christ (Student Edition) - Lee Strobel and Jane Vogel
The Case for Christianity Answer Book - Lee Strobel
77 FAQs about God and the Bible - Josh McDowell and Sean McDowel
Know Why You Believe - Paul E. Little.
Short and sweet inspirations for Christian teen girls.