The Christian Delusion: Why Faith Failsis the greatest critique of Christianity I've ever read. It is a sharp, thorough, and devastating case against the Christian religion. Not only that, but the book has contributions from several prominent atheists (Hector Avalos, Bob Price, Richard Carrier, John Loftus). This gives the book a very refreshing variety, as the contributors write on a wide-range of issues and look at them from different perspectives.
Part 1 is entitled "Why Faith Fails" and I take it to be a refutation of the common notion that one needs no rational reason to believe Christianity to be true, you can just "take it on faith." It is amply demonstrated that "faith" (in the sense that is used here) is completely unreliable, due to, (among other things) the fact that people "have faith" in a wide variety of religions, and since only one (at best) can be true, it makes one's own faith very likely to be false if the faith has not been rationally scrutinized.
Part 2 consists of three chapters which argue that the Bible is not God's word. Ed Babinski convincingly shows that the writers of the Hebrew Bible were given no divine revelation about the world in which they lived: their writings reflect belief in a flat earth covered by a dome (the 'firmament') which was the sky, or heavens. These beliefs are extremely and inexplicably strange if the Bible was inspired by God, but completely to be expected if the Bible was simply a product of its time and culture and not inspired by God. Paul Tobin, author of the website and book The Rejection of Pascal's Wager, explains a few of the contradictions and failed prophecies in the Bible, and I have a feeling that his approach would be very persuasive to fundamentalist Christians. Tobin proves his thesis far beyond a reasonable doubt, to the point that anyone unconvinced that the Bible contains inconsistencies after reading his chapter would have to be in deep, deep denial. Indeed, they would have to be in pathetic denial. John Loftus wraps up the section by discussing the "failure to communicate" of the Biblical God. The problem, as Loftus explains, is this: if God actually inspired the Bible, then why the hell is that Christians can't agree on the meaning of so many passages, and even on the basic messages of the biblical books themselves? Loftus' point could be forcefully illustrated by the following webpage, which lists passages in the Bible that condemn alcohol consumption followed by some that condone it. At the bottom of the page is a list of Christian websites that believe the Bible condemns drinking... Followed by a list of Christian websites that believe the Bible condones drinking! Well, which is it? And if God wrote it, why didn't he make his message clearer?
I don't have the time to review the other parts of the book in such detail, but the highlights of the rest of the book include: Hector Avalos' chapter arguing that atheism was not the cause of the holocaust or the mass murders of communist regimes and Richard Carrier's chapter on the Resurrection (which forcefully and convincingly argues that such an event, given the poor evidence we have, is totally irrational-- even illogical-- to believe).
To sum up, this book should be read by anyone who wants to understand the case against Christianity.