Everyone has a moment in history which belongs particularly to him. It is the moment when his emotions achieve their most powerful sway over him, and afterward when you say to this person "the world today" or "life" or "reality" he will assume that you mean this moment, even if it is fifty years past.
A Separate Peace by John Knowles takes place mostly in a summer session at Devon School during WWII. The boys at the boarding school are at the cusp of enlisting or being drafted into the military, but they have one last summer before the reality of war reaches them. Our main character, Gene, is reflecting on this time in his life 15 years before, when a boy names Phineas overwhelmed his life. Phineas is a fascinating boy, naturally athletic and friendly, willing to break rules, easily charming. But our narrator is Gene, so we view Phineas from the outside, knowing him only from how Gene felt about him.
Gene is an introverted character, revealing things to the reader that most people would keep private, like wanting the worst of your best friend, fancying some rivalry. Like showing how a shy personality can be taken in by a strong personality despite skepticism. Like showing how you can care so much for someone without ever telling them. Like honestly trying to figure out how to apologize and how to do right.
More than anything, the book captures the detached feeling of looking back a big moment in your life. It still holds power, but you've had time to look back at it and analyze, to realize what sort of an idiot you were at the time, but also to realize that the magic of that time can never be recaptured. A Separate Peace ached for that school year in the details remembered and forgotten by older Gene and how it would define his future.