SAM AND HIS BROTHER LEN by John Manderino. (Academy Chicago Publishers: $19.95; 234 pp.) Boys are so mysterious. In 1964, Len is 10, his brother, Sam, is 12. They fight the Japanese: "All right. Now, one more time," says Sam. "What are we fighting for?" "For God, and our country, and . . . I forget." "Peace in the world. Try it again. What are we fighting for?" They go to confession. Sam takes a long time. "What'd you do," his friend Dolan asks, "kill somebody?" "I had thoughts," says Sam. "About Sister." Sam's sport is hockey, Len's is baseball. They read Hamlet, Sam reads Ty Cobb's biography. Sam goes away to college where he listens to the Jefferson Airplane, and Len is lonely so he goes out with Doris. "When I get home for Easter," Sam writes, "I'll fill you in on the real story behind the war in Vietnam."