"If Alfred Hitchcock wrote novels, he would be herded into a roundup and thrown in the caboose at the back of the book section. If Agatha Christie made films, she would be the toast of France."

That's Wilfrid Sheed speaking, and not John Russel Taylor. But it does sort of put things into perspective, doesn't it? There always has been a marked contrast between form and content in Hitchcock's films, a disparity that may well be built right into the motion-picture medium. For after all, things that happen in the dark up there on that big screen happen to us; they involve the audience -- at least on the emotional level -- sometimes quite completely. It was because of Hitchcock's instinctive grasp of this essential truth about movies that he was able to exploit it so well. Like most geniuses, he was, according to Isaiah Berlin's designation, a hedgehog: cThe fox knows many things, but the hedgehog knows one big thing." And this critical biography of the director, in so many ways a model of its kind, is remarkable in that it tells us more about the many things he did not know than the one big thing he did.

In certain practical ways, Alfred Hitchcock's ignorance was appalling. The lower-middle-class son of a greengrocer, he grew up in a Catholic household and a Jesuit school where sexual repression was so much the order of the day that until he met the woman who was to be his wife, he had never even been out with a girl. When directing his first film, "The Pleasure Garden," at the age of 26, he had to be taken aside and told a few of the facts of life when one of his actresses refused to perform a stunt because it was "that time of the month."

He had managed to find his way into Britain's nascent motion-picture industry almost directly out of school. And once in, his drive, talent and attention to detail soon found him a place in the directorhs chair. From his first solid hit, "The Lodger," the Hitchcock style was established. The elements of that style singled out by Taylor are its "subtlety and moral ambiguity, as well as... the virtuoso display of sheer technique." And "The Lodger" was, of course, a thriller, his first, and ever after Hitchcock would work best, managing often to get surprisingly subtle effects, with just such material -- and such material only.

His was a craftsman's approach to movie storytelling, and he had a tecnician's sure sense of just what needed to be done to keep the action bubbling along. His catch-all word for such necessary plot elements was "MacGuffin." Every Hitchcock movie had its MacGuffin. But what precisely did he mean by it? I shall be eternally grateful to John Russell Taylor for providing a definitive answer to that in the form of this favorite shaggy dog story of the director's about two passengers on a Scottish train:

"There is a large mysteriously shaped parcel on the rack, and the inquisitive passenger asks the other what it is. 'A MacGuffin' is the reply. 'What's a MacGuffin?' 'It's for trapping lions in the Highlands.' 'But there are no lions in the Highlands.' 'Well then, there's no MacFuffin.'"

And so, as Taylo spells it out, "a MacGuffin is something totally irrelevant and nonexistent which is the subject of conversation and action...." It provides the picture with its plot, though not much else.

Reduced to such basic elements, Alfred Hitchcock's films seem perhaps both less and more than we may have first thought them. Less because taken on literary terms, there is, we realize, very little to movies that may have held us such as "The Thirty-Nine Steps," The Lady Vanishes" and "North by Northwest": They are pure MacGuffin. On the other hand, we also realize that although there was not much more to others such as "Strangers on a Train" and "Vertigo," they leave a disturbing sense of aftershock that lingers long after the plot's resolution. Hitchcock's genius was in managing, occasionally, to touch those deep wellsprings of emotion -- almost always he did this visually -- and set off those resounding aftershocks of feeling.

How did he do it? Why was he capable of it sometimes, though not at others? Was he ever fully aware of his ability to touch these dark places? These are the questions that John Russell Taylor doesn't answer. Oviously, I wish he had. Yet I certainly do not intend to feign dissatisfaction with a book so well written, one that provided me with so much pleasure. Although not perfect it is very good indeed. As a record of Alfred Hitchcock's life, it is, for the most part, a record of his films -- for the very good reason, Taylor would assure us, tht his films were his life. Well, perhaps. But think of Hitchcock's career as his MacGuffin, and you will realize that something is missing.