What it is, and what it isn't
As a non-Christian moving to the US soon, I wanted a good overview of Christianity from a believer. Not the complex history and the ancient world etc, but what it means to someone today. This book was the better reviewed of similar ones on Amazon.
It is a highly detailed overview of the Protestant sects of America today and in the past. It explains the meanings of Christian rites and the differences between many sects and how they are related. In some ways, it is probably better as a reference book than one you read straight through, due to its detail. It is an excellent view of what Christians believe, styles of worship, and major points of disagreement. It also somewhat addresses the question of why Christians seem uniquely focused on whether or not someone else is a Christian.
Special sections explaining many buzzwords, such as evangelical, born-again, and fundamentalist, are highly useful. There are also sections on specifically American groups: Seventh-Day Adventists, Christian Scientists, Mormons, Jehovah's Witnesses. I can't comment on the accuracy of those parts; sometimes it did feel like the author clearly was not in agreement, though the tone is always respectful and tries to be neutral.
That said, the book is highly focused on the American scene. Catholicism and the Eastern Orthodoxies are given light treatment relative to their populations, histories, and geographical spread (or maybe this is more noticeable to me coming from outside the US). They are often painted in broad strokes, while the slight differences between something like two kinds of Baptist may receive a whole chapter.
This book gave me exactly what I needed. Things it does not include are any kind of mysticism or personal experience anecdotes. It is not necessarily an accurate historio-sociological study of Christianity (except possibly for modern Protestant schisms), and there are certainly a few eye-roll inducing lines for the non-believer - historical accuracies are sacrificed and the Christian scriptures are presented as history a few times, and there are some pointed jabs that the "theory" of evolution isn't real, but overall a neutral tone is maintained. I wouldn't use this as an exclusive reference to Christianity, but it is a good guide to the American scene.