It comes as no surprise that this book (and books of its ilk) continues to receive such disparate reviews, as it either conforms to one's worldview, or it differs drastically. With that said, it took me some time to finally pen a review of my own - having read it a few weeks ago - because I wanted to do my best to take an objective approach to the material and give as unbiased an opinion as possible (something I think more reviewers should at least TRY to do).
In Letter to a Christian Nation, a rebuttal, of sorts, directed to the throngs of angry readers of his previous book The End of Faith, Sam Harris wastes no words, succinctly challenging the long-held Christian beliefs of this nation (to be more precise, the ultra-conservative Christian Right). In this pithy treatise, Harris debates what he considers to be many of the fallacies of the Christian faith, such as morality, by citing direct passages from the Bible, and pointing out their less-than-favorable intentions. Harris is often found quoting passages that demonstrate God's seemingly utter wrath and hatred of those who do not believe in Him, offering no punishment or tolerance for non-believers other than death, and juxtaposes it with the oft-lauded "Golden Rule." How can Christianity proselytize with both justified murder and treating others with respect? This is an obvious logical incongruity.
One has to give credit to Harris for doing his homework, as it is clear he spent considerable time with the Good Book before lambasting it (though, as he makes very clear, it is easy to cherry-pick the Bible to find supportive statements for any argument).
Harris certainly makes it difficult for Christians to defend many of the more deplorable aspects of their faith's history, and it seems unlikely he will accept any excuses. Harris obviously knows humans are fallible and prone to mistakes, but what he has no tolerance for is the continued ignorance and lack of progress of Christianity, especially on such issues as condom use and stem-cell research. Harris even argues, on the hot topic of abortion, that God is the most prolific abortionist in history, citing the numerous accounts of still-births and naturally-terminated pregnancies.
Harris' tone might be construed as smug, arrogant, and at times snide in nature (though, not to the degree of fellow "Four Horsemen" Christopher Hitchens), which could detract from his overall credibility. One can make arguments without resorting to blatant condescension. Perhaps it is just Harris' strong convictions permeating his writing, but as the saying goes, one attracts more flies with honey than vinegar.
(Actually, perhaps Harris has it right, using vinegar to attract more flies than honey, as his book has been read and reviewed by as many Christians as atheists - if not more - despite the obvious hatred he has for Christian beliefs.)
In the end, what Harris is really trying to preach is intellectual reasoning, leading the nation away from antiquated, primitive notions of how the world works. Faith can no longer be tolerated as a motive for one's actions or beliefs, and should be replaced with tangible evidence. Learn to question what you see and hear, and strive to continue learning, rather than accept that things are the way they are because it's "God's will."
Believe what you will, Letter to a Christian Nation is well-written, beautifully concise and should (though, unlikely will) challenge one's thoughts on the merits of the Christian religion. It will probably not sway you one direction or the other, but Harris will definitely be there to give you that extra little push.