Nicholas Eaton, one of the co-authors, was kind enough to send me an email about the book a couple of months ago.
The book is a collection of essays written by psychologists, all of whom picked certain aspects of BtVS, Angel, or Firefly/Serenity that they found interesting and readily discussable in a psychological framework. I think the "psychological gravity" you mentioned deals with how Joss's shows tend to have a certain psychological depth about them ( e.g., very good character development), which allows for worthwhile discussion and analysis. The chapter topics are really diverse, and they include such subjects as love, pornography, neuroscience, women's issues, mental disorders, personality, free will, and so on. I find the chapters (other than my own) to be really interesting reads. I think the overall purpose a lot of us had in contributing to this book was to help fans gain new insights into, and appreciation for, their favorite shows by viewing them in new ways; we also hoped to make psychology approachable and easily understood by a non-psychological audience. :)