Atheist Universe is an excellent introduction to the atheist point of view. It covers a wide ranging set of topics from a straight forward, "this is why we think that" perspective. If you're an atheist but not used to defending your turf, this book will help you see how do so. If you're wondering how someone can get along without believing in God, this book will explain it.
It isn't a perfect volume, though. Really deep explanations - the actual underlying science - isn't here. That's not a problem, really, and it would make the book vastly larger to include even a small portion of it. Mills summarizes it when needed, and he mostly gets it right. The years since the last update and the fact that I am better read them him in a few areas give me a couple of minor quibbles with his statements, but they don't invalidate his arguments.
More problematic is his style, which is somewhat "in your face". Some would call it aggressive or pushy. Others might call it calling a spade a spade. Regardless, he isn't afraid to tell you what he thinks is right, and in this era of political correctness I enjoyed much of it.
There is one chapter - on Christian fundamentalists and internet porn - which seems out of place to me, but the rest is pretty solid stuff. If you want deeper arguments - covering the philosophy or science in depth - you need to look elsewhere, but if you want a summary of why an atheist might think the things s/he does, this is a fine place to start. Just be prepared to be challenged if you come from that perspective. Mills is confident that all religion is silly and says so. I happen to agree with him.