The dangers of baseball
A key theme in this novel is the danger of baseball. Author John Grisham knows this from personal experience, as he was almost hit as a teenager, subsequently quitting the sport altogether.
In this novel, a young promising player called Joe Castle is hit in the head by a fastball, ruining the rest of his career. As such, we can see that the dangers of the sport, especially when players get jealous and competitive, is a key consideration of the author.
Forgiveness
When Warren is diagnosed with pancreatic cancer, Paul encourages him to seek forgiveness from Joe Castle, for the damage he has caused him after the accident. Through the novel, we learn that Warren was a jealous and frustrated player, who didn't like seeing Castle succeed. Warren is accused of deliberately "head-hunting" Castle, which is something Paul believes he must seek redemption for.
Father and Son
This novel also explores the relationships between father and son. Warren Tracey and his son Paul do not always see eye to eye, and this novel explores their complicated relationship and subsequent estrangement. Paul is deeply affected by the actions of his father and encourages him to make amends for them. When he realizes his father is dying of cancer, he reaches out in the hope of reconciling before he passes, and to help his father seek forgiveness from Joe Castle.