Honest, entertaining, significant
I first discovered "Ball Four" while on student exchange from Australia in the USA. Not knowing much about baseball, it provided me with a lot of very personal, open, and honest insight into the game.
It's written like a journal, so if you don't have a lot of time you can easily read very small sections without affecting continuity, but if you have all day there's nothing to stop you reading page after page.
"Ball Four" apparently caused quite a stir when it was originally released, because the American public didn't like to have the squeaky clean image of their heroes challenged. To me, it's refreshing to be able to get behind the scenes and see that the elevated superstars are ordinary people.