The single best book on the topic. Read it first, and go from there as your interest and serendipity allow. This is atheism from a scientist's point of view, brilliant, clear, and focused.

Americans more than citizens of other nations have to deal with Christian fundamentalism on a scale equal to or worse than the Islamic Fundamentalism in other parts of the world. Its only that, as Americans, its not so obvious how many parallels they have, nor how to combat the arguments of people who do not use reason and science.

Sam Harris, one of the Four Horseman (along with Dawkins, Hitchens, and Dennett), is popular and respected despite his young age compared to the older and dead supporters of atheism.

Together, the two Nietzsche books will give you the vast majority of man's thought.