What would be a good symbol to represent this book?A Separate Peace by John Knowles

While the river is an intended symbol of the division
between the carefree youth and the war, and the tree, also seems intended to symbolize
the breaking of friendship between Finny and Gene, in Chapter 1 of A Separate
Peace Gene Forrester as narrator describes the First Academy Building that
has a Latin inscription over the doorway that reads "Here Boys Come to Be Made Men,"
this doorway with the large cupola and a bell and a clock and its inscription could be
also be a  symbol, and, indeed, one that is representative of the
novel.

For, once Gene and Phineas, Brinker, Lepelier, and
the others pass through the doorway of the First Academy Building, they can never go
back to what they were before.  And, it is after passing through the door and answering
many bells and spending much time (the clock) at Devon that events happen to unalterably
change their lives.  For Leper, the War alters his life irrevocably as the sensitive
young man cannot cope with the rigors of the Army into which he has enlisted.  For
Phineas and Gene, it is the war in Gene's heart, the "something ignorant in the human
heart" that is the life-changing element as he imagines that Finny is jealous of his
academic ability and tries to make him fail.  Gene envies Finny his easy way, his
joie de vivre [delight in life] and his absolute lack of any petty
emotion, contriving out the "ignorance in his heart" a rivalry between him and Finny. 
It is only Finny who has the changes in his person imposed upon him as his heart does
not acquire envy or fear.  But, he does come to know that the "separate peace" he and
Gene experience on the afternoon of the Winter Carnival is special, but illusionary
because Gene has constructed an imaginary Maginot Line of defense between them, one that
leads to Finny's broken leg. Certainly, it is a different Gene Forrester who returns to
Devon School and walks again through the doorway of the First Academic Building in order
to understand what occurred within him at Devon School, a doorway through which he must
walk again in order to truly understand who he is, yet, always, one through which his
life has been irrevocably altered.