I recommended The God Delusion - I'm not sure if that's the best choice given the above intent, but it's what came to mind on the spot.

The problem with that book is not just that it will activate people's defense mechanisms, as some people have already noted, but that these defense mechanisms will often be, on the whole, a largely correct reaction. Significant parts of that book are nothing but ideological propaganda that attacks some metaphysical systems in favor of others, and when people get a visceral reaction that they're being propagandized and not enlightened by the writer, it's a correct intuition, even if it proceeds from spontaneous mental heuristics and not a rigorous logical analysis.

Even setting aside the issue that Dawkins is selling his own metaphysics in an underhanded way, as writers in this genre typically do, many of his arguments are full of errors of both logic and fact that clearly betray his ideological biases. (This is especially cringe-inducing when he takes a superficial and caricatured knowledge of history and colors it with his ideological preconceptions.)

(For full disclosure, I haven't read all of that book, but even if the remainder is much better, what I saw is enough to draw the above conclusions.)