In 1 Peter 3:15, we are told that we should always be prepared to give an answer for the hope that is within us. Today, we are bombarded with questions about the truthfulness of Christianity. These books will help you answer questions that you and your friends are asking.
The Reason for God by Tim Keller. Keller deals with questions like "There Can't Be Just One True Religion?" "How Could a Good God Allow Suffering?" "How Can A Loving God Send People to Hell?" and "Science Has Disproven Christianity."
A Shot of Faith to the Head by Mitch Stokes. The subtitle says it all: "Be a Confident Believer in an Age of Cranky Atheist." Like Keller (but with a different approach) Stokes also deals with the relationship of faith and science. Yet, this book is much broader than that. He also deals with the problem of evidence and the role of faith.
Tactics: A Game Plan for Discussing Your Christian Convictions by Greg Koukl. This book focuses on the practical ways you can engage with unbelievers.