Harris had written this book primarily for secularists so as to defend against the threats posed in the U.S. by the Christian Right. Unlike "The End of Faith", he says little about other religions, in particular Islam. This book was shorter and more about what I am familiar with.
Preaching to the choir? Undoubtedly to large extent but unlike "The End of Faith" I do not expect many secularists will feel he is unfairly extreme: this time he seems to be speaking well to address the concerns of many of us including those liberal and moderate Christians (and even conservative Christians) who do not feel well served by the political activities of the Christian Right.
Harris comments that "atheism" isn't any more necessary as a label than having to have "words for people who doubt that Elvis is still alive...". Anti-Elvisians? But would it be rude to the memory of Elvis, who, despite reported problems, was indeed a great performer? I did see and enjoy his movies. Perhaps Elvisism deserves founding, perhaps it already has been. Perhaps I could become a Elvisian apologist. Just show one of the movies or concerts and I'd have no trouble finding believers? Would you join? Pledge? Agree with me that Elvis not only lived but that I could speak for him. And that Elvis did not want your children learning about evolution in public schools. Or maybe gravity.
Foolish? Harris writes toward the end of this book "Clearly, it is time we learned to meet our emotional needs without embracing the preposterous". Harris expresses understanding: he writes that he does not "doubt that your acceptance of Christ may have coincided with positive changes in your life". He expresses no wish to discount those experiences but points out that many people now and before have had what seemed to him similar experiences in many ways, religious and otherwise. But he also believes that such experiences can be misinterpreted as one may do for those of other faiths. Claims that one's own religion is that different seems extraordinary and, as Carl Sagan popularized, "extraordinary claims require extraordinary evidence".
I, for one, can see no good reason why I would deny that Elvis lives if credible existence of that existed, especially if it promised me some life after death in Blue Hawaii, an opportunity to frolic with a youthful Ann-Margaret for eternity. I would gladly read the books of Elvisian apologists if I felt uncertain. It would all seem silly if there were not efforts underway at this moment in the U.S. to discredit evolution in public school science textbooks and even to establish a Christian nation with Old Testament laws.
Harris says it far better than I can, he's forceful but succinct (less than 100 pages). He concludes with a list of ten recommended book (I've only read five of them to date) that would make for good next reading steps. You may also consider registering with the Brights' Network (see their web site).