No God, But One, Evidence For Islam & Christianity, Allah or Jesus? by Nabeel Qureshi.
If you are going to have one book on understanding and responding to Muslims, you will be hard-pressed to do better than this. First rate apologetics, deep insights into the different mindset and worldview of Islamic thinking, short and concise chapters in a logical and easy to follow progression all make for a superb resource for anyone dealing with Islam or merely interested in understanding the religion.
5 stars. Highly recommended.