John Horgan's book is a tour through the intersection of science and mysticism. Of course, defining "mysticism" is not an easy task, and neither scholars nor the public have any sense of agreement of what this terms should mean. For Horgan, it means non-traditional, perhaps esoteric (an older term), views of reality. Horgan begins his tour with the doyen of religious studies, the venerable Huston Smith, and then moves on the the grand-synthesizer, Ken Wilber. Both of these men are big picture thinkers. From these high-altitude views (although both are practitioners as well), Horgan tours various thinkers like James Austin, Andrew Newberg, S. Grof, Susan Blackmore, and others. Each as a different take; none can provide a final, definitive answer. It's all too big, in one sense.
Horgan's tour is worthwhile, as he is at once inquisitive and skeptical; scientific (he's a science writer) and a seeker. In the end, he gives us no final answers, but more important and worthwhile questions. In a sense, we leave this book as much seekers as when we started, but with a few more insights.