In 1981, two fledgling wine writers snagged their first assignments from Food & Wine magazine. One wrote about a kosher winery on the Lower East Side and is now your Urban Vintage columnist. The other rated jug wines and became the most powerful wine critic the world has ever known. His name is Robert M. Parker Jr., and he is the subject of a new biography by Elin McCoy, “The Emperor of Wine: The Rise of Robert M. Parker, Jr., and the Reign of American Taste” (Ecco, $25.95), due out at the end of the month. It was Ms. McCoy, along with her co-editor John Walker, who assigned those two stories at Food & Wine.

Along with charting the remarkable ascent of Mr. Parker, Ms. McCoy’s book offers an excellent big-picture view of the prickly world of wine politics. Jonathan Nossiter tried to provide such a view earlier this year in his film “Mondovino,” but his anti-globalist anger interfered with his ambition.

The seemingly modest source of Mr. Parker’s power is the Wine Advocate, a newsletter prepared at his home in Parkton, Md. It appears bimonthly, costs $60 for a subscription, and currently has a relatively paltry paid circulation of 40,000. What gave the Wine Advocate its huge leverage early on was Mr. Parker’s simple yet brilliant decision to create a 100-point scale for rating wines. No wine critic before him had presumed to do that. Readers intimidated by wine mumbo-jumbo had no problem understanding a “grade” for a particular bottle exactly like their high school test scores. Ms. McCoy sums up the cascade of benefits that a high Parker score could set in motion: “Producers used them to sell wine to importers, who used them to convince distributors to stock them, who used them to get retailers to buy, who posted them to impress dinner guests. Numbers were the new way to market wines. And Parker was the first generator of the numbers.”

Mr. Parker’s rise owed nothing to birthright. Nobody in his family, owners of a dairy farm a few miles from where he now lives in Parkton, had any interest in wine. Nor did Mr. Parker himself, until he visited his wife-to-be, Pat, in Paris, where she was spending her junior year abroad. On their first evening at a bistro, the budget-strapped Mr. Parker discovered that a glass of wine cost less than a Coke. And thus began his affair with wine.

Mr. Parker earned his spurs the hard way. As a law student and then as a lawyer for Farm Credit Banks, he lived for the after-hours when he could haunt wine shops, buy wine, and open bottle upon bottle with friends in comparative tastings. In 1978, the first edition of his newsletter appeared, then called the Baltimore-Washington Wine Advocate. Mr. Parker wanted to alert wine buyers to smart buys. And he was only too happy, Ralph Nader-style, to expose famous and overpriced wines, especially hallowed Bordeaux and Burgundies that were coasting on their names. Another potent incentive for starting the newsletter, Ms. McCoy points out, was his wife’s dismay at his enormous expenditures on wine. Now every penny would become a tax deduction.

The time was right for Mr. Parker to ride a white horse into the wine world. Middle-class Americans were traveling to Europe and experiencing the pleasures of sophisticated dining with wine. Julia Child had turned them on to French cooking at home. She gave them confidence to make boeuf bourguignon or coq au vin. But what about the wine? Mr. Parker came to the rescue with his rating scale. How great to discover a $6 Cotes du Rhone that was better than a $25 Bordeaux!

Other wine-appraising publications were out there, of course, and many of them switched to Mr. Parker’s 100-point scale. But the race went to Mr. Parker, who seemed best equipped to forge ahead. He had, for one thing, enormous self-confidence. Mr. Parker’s career really took off when he successfully predicted that the 1982 vintage in Bordeaux would be a great one; other critics were less enthusiastic, and followers of Mr. Parker’s advice made a fortune when he was proved right and prices for 1982 Bordeaux soared.

Mr. Parker also had a French-speaking wife who, during his early visits to France when he had yet to learn the language, both translated for him and charmed winemakers. And then there was his exceptional nose and palate.

Ms. McCoy precisely describes the nitty-gritty of Mr. Parker’s work: marathon tastings conducted in front of a deep sink in a special room at home or along the wine route, in which he tastes and spits one wine after another, focusing intensely before assigning a number, and never looking back. We learn that eating garlic on the evening before tasting doesn’t throw off his tastebuds, but watercress does.

Ms. McCoy closely observed Mr. Parker as he made premium winery rounds in the Napa Valley one spring day. And a very long day it was, although Mr. Parker ate only bananas and drank bottled water while tasting. Starting at 7:30 a.m., he tasted wines at Staglin Family Vineyard, Flora Springs, Patz & Hall, Signorello, and Shafer Vineyards. At 6:30 p.m., when they returned to Meadowood, the exclusive resort where Mr. Parker was staying, he was “still going strong.” The day was not over, since he was still due to meet Piedmontese super-vintner Angelo Gaja, who wanted him to sample a new line of ultra-premium wines. The most illuminating interplay of the day was between Mr. Parker and Julie Garvey, a soft-spoken but fearless co-owner of Flora Springs winery. So many vintners genuflect before the emperor, but Ms. Garvey chided him for past criticism as he tasted. “He thought [our wines] were too elegant,” she told Ms. McCoy while eyeing Mr. Parker.

“No, I thought they lacked texture and I thought they were too acidic,” he replied.

“Um-hum. But the word you used was ‘elegant.'”

“Well, too politically correct, too elegant, too polite. …”

Ms. Garvey kept the pressure on, reminding the Mr. Parker, “You said ‘death to the winemaker.'”

“That is shameful conduct on my part. … You’re sure I said that? I must have really been pissed off tasting those wines.”

Ms. Garvey’s moxie is the exception to the natural tendency of vintners to make nice to Mr. Parker, at least to his face. And the exchange between them addresses a major criticism of Mr. Parker most often heard from the French: His predilection for rich, ripe wines with oodles of fruit works unfairly against subtle, earthy wines that are “too elegant, too polite.” (Mr. Parker objects to this characterization, and tells Ms. McCoy so in the book.)

Having thoroughly examined her subject, Ms. McCoy wraps up her book by standing back to pose the inevitable big question: Has Robert Parker’s influence on wine been constructive or destructive? In answering, she’s of two minds. On the one hand, “His scoring system … has encouraged the idea that only top-rated wines are worth drinking. On the other hand, “his insistence that no reputation, no matter how longstanding, can substitute for what can be found in the glass … and his fundamental conviction that wine can be judged and rated like anything else are lasting contributions.”

My own feeling, having never met Mr. Parker, is that his enormous and inextinguishable passion for wine is all to the good. His preferences may not be mine or yours, but that’s what makes wine so interesting.

It’s to Ms. McCoy’s credit that her portrait of Mr. Parker is finally so complex, so riddled with his contradictions, and so suffused with his enormous energy, that her subject is never cornered.