The book is a diary of Bouton's 1969 season, spent with the Seattle Pilots and then the Houston Astros following a late-season trade.

[1] Ball Four described a side of baseball that was previously unseen by writing about the obscene jokes, drunken womanizing, and routine drug use among players, including by Bouton himself.

Bouton additionally described clashes with his coaches (usually about his role with the team, his opinion that he should use the knuckleball exclusively, and his desire to throw between outings) and his outspoken views on politics.

[5] The book's title was suggested by a female customer of a tavern called the Lion's Head in New York City's Greenwich Village neighborhood.

The book made Bouton unpopular with many players, coaches and officials on other teams as well, as they felt he had betrayed the long-standing rule: "What you see here, what you say here, what you do here, let it stay here."

[9] Hank Aaron, Leo Durocher, Mickey Mantle and Tom Gorman, each of whom had, at one time or another, been either directly or indirectly associated with Bouton, expressed their opinions on the book, none of them favorable, on a 1979 episode of The Dick Cavett Show.

[10] The following year, Bouton described the fallout from Ball Four and his ensuing battles with Commissioner Kuhn and others in another book, titled I'm Glad You Didn't Take It Personally.

[11] Bouton starred as "Jim Barton", a baseball player who was also a writer with a preoccupation with his teammates' personal lives.

But Posnanski says the book is really about trying to hold on to youth, citing the last line: "You spend a good piece of your life gripping a baseball, and in the end, it turns out that it was the other way around all the time.