Philosopher Julian Baggini, an editor of Philosophers' Magazine and a contributor to Butterflies and Wheels ("fighting fashionable nonsense"), has written a necessarily brief but very useful and engaging book, Atheism: A Very Short Introduction.  One of the Very Short Introductions series, it fits easily in your back pocket, a nice little red book of atheism that covers a good deal more than just the basics, with suggestions for further reading.  Baggini is a gifted writer - precise, even-handed, insightful - who puts atheism in the wider philosophical context of science, physicalism, rationalism (small-r) and naturalism.  He shows atheism to be a consequence of naturalism, which is in turn based in a commitment to myth-supplanting rationality, "which confines itself to reasons, evidence and arguments, that are open to scrutiny, assessment, acceptance or rejection, on the basis of principles and facts which are available to all."

Baggini addresses the big questions of ethics, meaning, and purpose, showing that atheists are at no disadvantage in being able to live meaningful and moral lives.  Although he makes an excellent case for atheism as the mature, reality-based acceptance that we inhabit a natural universe unsupervised by a supernatural intelligence, he wisely refrains from taking a militant anti-religious stance.  Those of us with a strong preference for evidence-based beliefs will not win converts by contemptuous assaults on theistic assumptions.  As he puts it, "Religion will recede not by atheists shouting condemnation, but by the quiet voice of reason slowly making itself heard."  I would only suggest that it isn't really religion that should recede, since there are religious naturalists out there who share Baggini's commitment to evidence and rationality.  Rather, it's non-evidential faith that stands most opposed to a naturalistic enlightenment.  And indeed it's faith that Baggini most calls into question.  Despite its brevity, his book is a well-crafted, illuminating resource for those seeking to champion a humane atheism.