This is a complex book and restricting myself to two or three adjectives is difficult!
The first adjective I would choose is "nostalgic." This is a book about a man looking back on an important time in his past. While that past involved tragic events, there was joy in that period of Gene's life, the time before he had to become a responsible adult. The fact that he was expected to go to war when he left school gave this period particular importance to him, and there is a tone throughout the book of nostalgia.
The second adjective I would choose is "conflicted." While the story takes place in a separate, peaceful time outside the reality of war, there is much conflict in the book. Gene has internal conflict in his feelings about Finney, admiring him and envying him simultaneously. He has conflicted feelings about the war, and he has conflicted feelings about his actions, as well. Gene's ambivalence about life, maturity, friendship, and war is demonstrated throughout in the tone of the book.
My final adjective is "claustrophobic." That may seem like an unusual choice, but think about the isolation of these boys, all together on one campus, having to deal only with one another day in and day out, with few places to escape to, eating meals together, studying together, attending class together, and having all athletic and recreational activities together. This claustrophobic atmosphere and tone cause all the events in the novel to be even more significant in the eyes of the characters and the reader.
A Separate Peace is a book that examines the coming of age of its two central characters, Gene and Phineas. It is an adventurous book that takes a daring look at the nature of friendship in a world dominated by war and uncertainty.
Phineas and Gene reach maturity under the cloud of their impending military service in World War II, there is an enchanting feeling at the school, as if nothing from the outside can touch them. Devon serves as a shelter for these boys, until the tragic incident which redefines the relationship of the two boys.
Gene struggles with Phineas's manipulative personality, trying to grow into his own perspectives on life while maintaining a relationship with a boy who is a leader, capable of inspiring others to follow him. In particular, Gene suffers from a degree of deeply held insecurity and a bit of jealously toward his friend's outgoing, brave, calm, courageous personality.