I choose to review this as journalism, although the author is famous for Creative Nonfiction.
This book was written at a time (1974) of historical changes in baseball, among both players and umpires. Only a few years earlier, Umpires had to strike to achieve benefits and fair wages. A decision from that year (Curt Flood) paved the way for free agency in 1975. One member of the crew that the author followed was the only African American umpire in all of baseball, and the first in the National League. So this book rates very highly for it's source material alone.
The style, unfortunately, is not history but a very laid back journalism. The author says as much in his 1999 preface to this edition. Even going into the book with that expectation, I was left wanting more facts, more history, less word-for-word reporting.
I sought this book out (thank you Albequerque and inter-library loan!) because it was listed in a collection of baseball books worth reading, in the umpires section. It does cover the umpires point of view, but other books have done better in this respect. What this book does have to offer is a history of baseball and umpiring in the early seventies.