I read this book last fall and quite enjoyed it. The author more or less wraps his comparative religion class at Boston University inside the broad them of pushing back against the notion that all religions are really the same in some essential sense.
The Washington Post reviewer does a good job of capturing the book. As he puts it:
Intellectuals friendly to religion have fostered an equally misleading notion, one that is thoughtfully dispelled in Stephen Prothero's book, "God is Not One." Seeing the world's major belief systems through Enlightenment-tinted glasses, a succession of influential philosophers, artists, scholars and even many religious leaders have tended to minimize the differences of ritual and dogma among the various religions to emphasize a supposedly universal and benign truth shared by them all. Such well-meaning believers (and they do constitute a kind of religion of their own) have subscribed to variations of the Dalai Lama's affirmation that "the essential message of all religions is very much the same."
The reviewer also notes Prothero's take on evangelical atheists like Christopher Hitchens:
And not only friends of religion abuse the truth through such generalizing, says Prothero. Richard Dawkins, Christopher Hitchens and other so-called New Atheists attack religions as if they were an undifferentiated mass of barbaric superstitions, all having a disastrous effect on the development of humanity, rational discourse and civil society. Prothero does not deny the evils that have been done in the name of God; he insists that it is precisely a religion's mixture of dark and light, its potential for good and evil, that makes each and every religion so distinctive and so ineradicably human.
The message I took away was: "within" variation is really important. In the US, we live in a sea mainly of Christian denominations, and have a sense that they vary a lot, from bible-thumping fundamentalists to intellectual Unitarians to wishy-washy do-gooding Episcopalians, not to mention the distinctly American traditions such as Mormonism, Jehovah's Witnesses and New Thought. But because we do not have them around us as much, it is easy to forget that other traditions, such as Islam or Hinduism, have at least this much variation, and that the variation is on multiple dimensions, not just the one-dimensional liberal versus conservative dichotomy favored in media treatments.
I also quite enjoyed learning more about the traditions I knew least: Hinduism and the Yoruba religions of Africa. Plus there are some funny stories about the author's comparative religion students. One of the assignments he gives them in his course is to design their own religion, the results of which, as he reports them, can be both funny and moving.