The Varieties of Scientific Experience: A Personal View of the Search For God by the late Cornell University astronomer Carl Sagan has much the same premise as Sam Harris' Letter To A Christian Nation, a book I blogged about some time ago. However, I think it's safe to say that these two books offer profoundly different reading experiences.

The confrontational tone of
Letter is bound to rile you up, whether you are a believer or nonbeliever, and its format (a letter from him to you) precludes the idea of dialogue and results in a declaration of the author's views. As such,
Letter is very effective at generating a visceral response, whichever side of the fence you are on, and this might explain why it was a best seller.
The Varieties of Scientific Experience, on the other hand, adopts a different approach altogether. The book is a transcript of a series of Gifford Lectures on Natural Theology that he delivered to live audiences at the University of Glasgow in 1985. The scientific method guides his various arguments: "...where we are asking about the truth, we ought to be sure that it's not inconsistent with what else we know.
We should also pay attention to how badly we want to believe a given contention. The more badly we want to believe it, the more skeptical we have to be. It involves a kind of courageous self-discipline." Overall, the tone of this book is well-reasoned and yet infinitely gentle.
This gentleness derives from Sagan's rationalism as well as from his personality, which was kind, humorous, and interested in connections, both scientific and interpersonal. This sense of connection with his audience is underscored by the lengthy Q&A excerpts that appear at the end of the book. The questions were often from the faithful and his responses were always straight but never demeaning.
So what does he say? Well, he touches on a number of topics: humility deriving from the vastness of nature (including and especially the cosmos), the probability of life elsewhere in the universe, the various interpretations (including his own) of religious experience, and the many and conflicting definitions of "God": "...my proposal is that we call reality "reality," that we call love "love," and not call either of them God, which has, while an enormous number of other meanings, not exactly those meanings."
Towards the middle of The Varieties of Scientific Experience, Carl Sagan quotes a poem from an acquaintance that he believed sums up the essential feeling that underlies human religious experience. His response was telling: "...if we are merely matter intricately assembled, is this really demeaning? If there's nothing in here but atoms, does that make us less or does that make matter more?"
You decide.