I found it in a used bookstore in which I had store credit. On the surface, it's about two boys in a New England boarding school during World War 2. One day while they're playing their daredevil game of jumping out of a tree into a river, Gene, the narrator, intentionally shakes the branch so that Phinneas falls out of the tree and onto the ground, breaking his leg. It was a passing thought that materialized without complete consciousness, but it was borned out of a combination of Gene's jealousy of Finny's athletic ability and popularity. Through the rest of the book, they ignore and then finally deal with the situation.
For a long time, they're both in denial, and they treat the war the same way. Finny seems to believe that the war is just a ruse, put on by the old fat men of the country so they can laugh at everyone else. And for a while, they believe that. (It's their "Separate Peace".) But eventually they have to come to terms: Finny gets mad at Gene, and Gene apologizes, finally feeling guilty. In a classic case of paralellism, the novel ends with Finny falling again and re-breaking the leg. Just before he dies, he forgives Gene.