Robin Le Poidevin has written one of the best books defending atheism. He fairly and accurately considers his opponent's arguments, yet he still goes through meticulously and points out the flaws in each one.
The book is divided into three parts. Part one should be the most interesting to the atheist. Poidevin discusses theistic explanations, like the cosmological and teleological arguments, and finds them to be wanting. His discussion is fair and balanced, and he takes interesting approaches to dealing with them (for example, his discussion of modal realism in regards to the ontological argument).
In part two Poidevin attempts to make arguments *for* atheism, instead of simply refuting theistic claims. This section may appeal less to the atheist and theist than to someone who is still "unsure." I found his treatment of the problem of evil (AE) to be interesting, yet he claims that it is "the most powerful argument for atheism," which is a bit of an overstatement. At best, AE offers support to an atheist, but it is not the knock-down argument Poidevin makes it out to be. His other argument "for" atheism is really just pointing out the problems with theistic ethics. He states the Euthyphro dilemma, then discusses the ethics in much greater depth. I won't go into it here, but although his discussion was interesting, I am not sure it is really an argument "for" atheism.
Part three will probably not interest the atheist all, but the theist may find it more interesting than parts 1 or 2. As an atheist, I found all 3 parts to be interesting, and part 3 especially so for its treatment of difficult issues. Part three asks if religion may have pragmatic value (the answer is yes), and if so, can that be salvaged by the atheist (Poidevin's answer again is yes). Part three also deals with the issues of death and whether we can have meaningful discourse about God at all. All in all, I found part 3 the most interesting because it deals with issues that most theists find more important than the newest statement of the teleological argument: Should the atheist fear death? How can the atheist salvage the comfort of religion from the fact that god is a fiction? etc. These issues often don't appeal to the person who already is an atheist, but Poidevin's treatment of them shows his breadth of understanding about issues that the atheist faces.
One final note: This book is not extremely difficult, but I do not agree with another reviewer here that said it is accessible to the non-philosopher. If you've never read any philosophy before, you will probably find this at least minimally difficult. Poidevin defines terms beforehand, and he restates things often enough to keep most people from getting lost, but the preface of the book states that it is written for a "second or third year undergraduate philosophy of religion or metaphysics course." Some background in philosophy will make this book more enjoyable and you'll get more out of it.
All in all, this is so far the best book arguing for atheism that I have ever read. It deals with issues that are important to both atheists and theists, and Poidevin treats his opponents with fairness. This text is simply a must have for anyone interested in the philosophy of religion.