There are several highlights to John Grisham's first novel about baseball.
The first is Grisham's love for the game. He's a longtime fan, and dreamed of being a major league player when he was a kid.
His knowledge of baseball comes through in his choice of subjects. Ever since Ray Chapman was killed by a pitch, fans have known just how risky it is for batters to face pitches thrown at major league speed. That's what happens to Calico Joe Castle, who the novel is built around. So, the first highlights are knowledge and love.
As far as the content, the main highlight are the relationships in the novel (between father and son, and Joe's relationship with the community), the moral quality of the book, and way you sympathize with Calico Joe's dreams and disappointment.