This is the book "Harvard Psychology Lectures" (Daiwa Bunko) by Brian R. Little, translated by Osamu Kojima. The original title is "ME, MYSELF, AND US," and it's an introductory book on personality psychology. The theme is that "knowing yourself" is your greatest strength, and it aims to discover the "hidden self" that is not just determined by genetics and environment. This is a paperback version of a bestseller that has sold over 100,000 copies. It explains that deeply understanding your multifaceted self will be a weapon to make the most of your strengths, understand others, and live a better life. I've only read it once.