What is the theme in The Catcher Was a Spy: The Mysterious Life of Moe Berg by Nicholas Dawidoff?

Low Self-Esteem is a theme in the book.

Moe Berg would always be mediocre. When he became a professional baseball player, Moe had the stuff to be a star, but he purposely sabotaged himself by returning late for spring training and performing badly and later by attending law school, a decision that forced him to miss whole sections of the regular season. Later, when Moe began to shine as a catcher, he injured his knee and caused himself to be permanently relegated to third stringer.