A high school football player who considers himself to be a loser longs for the company of the prettiest girl at his high school, but has such low self-esteem he can't believe she would like him. The girl, though, knows that if he would simply believe in himself he could find that she does like him. A high-school novel from the late 1970's.
Severity reflects intensity, not value — “central theme” means a warning is a core part of the book, not that the book is bad.