Second for The God Delusion. I read it in 2006, and it provided a good primer for at least helping me understand what atheism really is. I did not declare myself as such right there and then, however.
Really, it was looking at several lines of evidence, including history, biology, physics, human psychology, etc. that convinced me no omnipotent god existed. This conclusion took about another 5 years to reach.
My point is, a single book will not do it. Those with theistic upbringings, do not easily lose those tendencies toward belief, since certain existential anxieties often need to be dealt with first. This takes time. Those with fundamentalist upbringings, (such as JW), are much more susceptible to such anxieties, as religion and belief in god tend to have addictive properties, that can be difficult to break free from.
d4g