All the Broken Pieces
a novel in verse by Ann E. Burg
Given up by his mother in Vietnam, Matt is adopted by an American family where he learns to play the piano and baseball. He has the luxury of a relationship with Vietnamese veterans, including the talented man who is teaching him piano, to help him process and heal from his memories of his war-torn homeland. He carries a secret guilt about an accident with his younger brother in Vietnam that threatens to destroy his new life until he can share it with his new parents.
This is an excellent book of war, baseball, and healing, which would be especially attractive to a middle grade or middle school book who loves baseball and likes a page without too many words on it (because it's in verse rather than prose).