Writer Bruce Weber went to umpire school and has a new book out about it. Sounds good:
The conundrum of umpiring is neatly posed by the peculiar rule of the knee-to-shoulder strike zone, which Mr. Weber calls the sole instance of a playing area that is demarcated only in the minds of officials. “The strike zone isn’t, nor has it ever been, set in stone, or even sand,” he writes. “It’s set in air, a concept, not a thing. It can’t be transported from one ballpark to another, but like the memory of a secret code it has to be formulated by each umpire each time he squats behind the catcher, every game, every pitch.”
One umpire compares the strike zone to a Supreme Court ruling: the language may be clear, but generations will reinterpret it.