The book starts off defining an observable phenomena, talking about the observables (physically, psychology, etc.) that combine to define the phenomena, and then utilize this definition to look at cases which observe and do not observe the phenomena. Due to this the book does not fall into the trapping of the including the kitchen sink to expand the definition to fit everything. Instead we take a more scientific approach to see the cases which do or do not fall under these definitions, through time and culture, and with or without the loaded vocabulary. The same as can be said for religious phenomena such as that observed by "The Idea of Holy" by Rudolf Otter.
Now that we are talking about a defined (as much as can be) phenomena with observable traits then proper research can be conducted without the (ab)use of loaded words such as "possession". The book includes numerous references for further research including direct sources, research papers with DOI's, studies, books, and more. These including, of course, a large multitude of sources to study and form opinions on.
Highly recommended for anyone into this topic who is looking to understand and without falling into the kitchen sink issue of "possession".