Here is the salient point about authorized biography: The biographer has access to everything his subject wants him to know. No matter how frank the subject, the material has, in a sense, been preselected. In Alec Guinness’s case, this means Piers Paul Read has seen the Guinness diaries, journals, and correspondence, and if any Guinness family member, friend, or colleague has given the biographer grief, we are not told about it.

The result is, on the one hand, a deep look into Guinness’s psyche, especially his ambiguous sexuality, his enduring if rather brutal marriage, and his domestic life and professional career. I cannot recall a biographer who has rendered the subject’s view of himself and what others thought of him so well as Mr. Read – unless it is Boswell himself.

The comparison is apt, since like Boswell, Mr. Read met his subject in the latter stages of the subject’s life. And, like Boswell, Mr. Read balances affinity with perspective: We see Guinness both in close-up and in the round. Mr. Read shares Guinness’s Catholicism, and perhaps as a consequence Guinness is portrayed as a sort of saint manqué. All that is missing really is the performance of a few miracles.

Guinness, a man tormented by lascivious thoughts and perturbations about his unworthiness, sometimes went to Mass every day and took communion twice a week. He sought confessors throughout the world. He realized he could not achieve spiritual perfection, but that did not make him any less demanding of his friends, his wife Merula, and his son Matthew. Guinness was a tidy soul and on occasion publicly berated his less fastidious wife and son. He was more than a bit of a misogynist.

Was he also a homosexual? That he was attracted to young men is amply proven in this biography. But I like the way Mr. Read hedges. He cannot demonstrate, for all his access to the Guinness archive, that his subject ever sexually consummated his desires for men. Indeed, Mr. Read believes that Guinness’s spiritual discipline, his monumental effort not to act upon his homosexual desire, is one of the key ingredients in the power his subject brought to acting.

Judging by this biography, Guinness’s greatest screen performance was probably as George Smiley (John Le Carre’s master spy) in 1979’s “Tinker,Tailor,Soldier, Spy,” a worthy companion to the stage triumph of his first “Hamlet,” which actress Joan Wyndham described in her diary of 1938:

Not ranting and roaring, but moving quiet realism – with touches of elfin humour, and at times, an unbearable pathos, as of something lost and bewildered. He had the quality of being able to stand still and yet arrest the attention. Slow unhurried moves, and then stillness. Nothing fussy or unnecessary.

Perhaps it takes an actor to provide this kind of description. Certainly it is beyond Mr. Read’s range. The biographer dutifully reports what others thought of Guinness’s roles and the subject’s own often ruthless estimation of his talents, but missing from this long biography is any sense that Mr. Read has seen the films and has any idea of what constitutes a good performance.

To be sure Mr. Read provides some behind-the-scenes scenes of what went on when Guinness performed in plays and films. But what is Mr. Guinness’s repertoire of skills as an actor? What distinguishes him from his great contemporaries, Laurence Olivier, John Gielgud, and Ralph Richardson? Occasionally Mr. Read quotes a critic who makes a few comparisons, but this won’t do.

Acting was central to Alec Guinness’s life, and acting is a creation of self as much as autobiography or any other form of writing can be. The biographer who does not have the equipment to describe the process by which one becomes an actor is not doing his job. I kept wondering: Did Guinness not leave any notes at all – however fragmentary – about how he would play his roles? Did he never comment on the autobiographical implications of choosing to play certain parts? Only when he turns down the role of Aschenbach, the composer in the film version of “Death in Venice,” does Mr. Read even begin to hazard some thoughts on how this character attracted to young men might have been uncomfortably close to Guinness’s own predilections.

The fact is that, though he published memoirs and left behind various bits of unpublished writing, Alec Guinness did suppress a good deal of what he felt. Mr. Read, unfortunately, confines himself to a rather small circle of Guinness’s friends and fellow actors and does not – as an unauthorized biographer might – cast a larger net.

The reader must turn to Garry O’Connor’s “Alec Guinness: The Unknown” (Applause Books) to confront the question of Guinness’s acting more squarely. Mr. O’Connor interviewed Guinness, who then abruptly withdrew from cooperation, confessing that he had probably said something indiscreet. Mr. Read, unlike Mr. O’Connor, shies away from blunt questions, such as “Was Guinness a great actor?”

Biography is an indiscreet act – or, as Boswell put it, “a presumptuous task.” Authorized biographers too often behave like apostles and come too close to sharing their subjects’ faith. This biography’s greatest strength is Mr. Read’s understanding of Guinness’s reticence. Its greatest weakness is not to know that biography itself also requires a certain rudeness.