Look at the work of Harvard Professor Jerome Kagan, or the fRMI work of Carl Schwartz at MA General Hospital. People are different, and the differences are measurable, both behaviorally and in brain structure.. It's not as simple as introversion-extroversion, but its also not a useless way of thinking about it.

Anyway, somebody offered you a book to read. You chose to call the ideas in it ludicrous without reading it. Not sure why.