In Mark Twain's "A Connecticut Yankee in King Arthur's Court," a time-traveling American brought baseball to sixth century England, where arguments with umpires were robust: "The umpire's first decision was usually his last. … When it was noticed that no umpire ever survived a game, umpiring got to be unpopular."

But it remains a necessary, extraordinarily demanding and insufficiently appreciated craft.

Now, however, comes "As They See 'Em: A Fan's Travels in the Land of Umpires" (Scribner, $26, 352 pages) by Bruce Weber of the New York Times. Forests are felled to produce baseball books, about 600 a year, most of them not worth the paper they should never have been printed on. Weber's, however, is a terrific introduction to, among much else, the rule book's Talmudic subtleties, such as:

A great fielding play can cost the fielder's team the game. With less than two out, if a player makes a catch and falls into the stands, every runner moves up a base. So with a runner on third in the bottom of the ninth of a tie game, if a fielder makes a catch but his momentum flips him over the railing into the seats, his team loses.

Also: There is a play on which the umpire must give a manager a choice of two different outcomes on a batted ball. With one out and runners on first and third, the batter swings, his bat ticks the catcher's glove but drives a fly ball that is caught by an outfielder. The runner on third tags and scores, the runner on first stays there. But because the catcher interfered with the batter's swing, the umpire awards the batter first base, moving the runner there to second. Because that nullifies the sacrifice fly, the runner who scored is returned to third. But why should the batting team lose a run because the other team's catcher committed an infraction? So the manager of the team at bat is given a choice – bases loaded, one out, no run in, or man on first, two out, one run in.

Umpires – the only people who are on the field during the entire game and the only ones indifferent to the outcome – were depicted in pre-Civil War drawings wearing top hats and carrying walking sticks. An account of the (supposedly) first game between organized teams – June 19, 1846, in Hoboken, N.J. – mentioned the umpire fining a player 6 cents for swearing.

Umpires still are custodians of decorum: "As the umpire," Weber writes, "you are neither inside the game, as the players are, nor outside it among the fans, but … the game passes through you, like rainwater through a filter, and … your job is to influence it for the better, to strain out the impurities."

Baseball is, Weber notes, the only sport that asks an on-field official to demarcate the most important aspect of the field of play – the strike zone. Although defined in the rule book, its precise dimensions are determined daily by the home plate umpire.

Umpires are islands of exemption from America's obsessive lawyering: As has been said, three strikes and you're out – the best lawyer can't help you. But because it is the national pastime of a litigious nation, baseball is the only sport in which a non-player is allowed onto the field to argue against rulings.

Umpires are used to having their eyesight questioned – when someone criticized Bruce Froemming's, he said, "The sun is 93 million miles away, and I can see that" – but their integrity is unquestioned. As Weber notes, players, not umpires, conspired to fix the 1919 World Series; a manager (Pete Rose), not an umpire, was banned from baseball for betting on games. As umpires say, "If they played by the honor system, they wouldn't need us."

Sport – strenuous exertion structured and restrained by rules – replicates the challenges of political freedom. Umpires, baseball's judicial branch, embody what any society always needs and what America, in its current financial disarray, craves – regulated striving that, by preventing ordered competition from descending into chaos, enables excellence to prevail.

"You can't," Weber says, "hide on a baseball field."

But a batter who fails two-thirds of the time for 15 years goes to Cooperstown. An umpire can fail once in a high-stakes moment and be remembered for that forever. It is amazing how rarely they fail as they strive not to be noticed in their pursuit of unobtrusive perfection.