From Gielgud, mellow memoirs offered with relish and good humor; Gielgud: An Actor and His Time, A Memoir, by John Gielgud in collaboration with John Miller and John Powell. New York: Clarkson N. Potter Inc. $14.95.

John Gielgud gazes musingly out of the dust jacket. His smile is both benign and quizzical. Therole portrayed is Eminent Actor as Author, and Sir John assumes it as naturally as any he has played on stage or screen, The understated theatrical flair of the pose assures the reader that "Gielgud: An Actor and His Time" is going to be a bona fide theatrical memoir -- rich in achievement, reminiscence, anecdote, and illustration (200 photographs including a dozen in color).

Reassurance comes with reading. The character of the book is explained in Gielgud's brief foreword. The manuscript evolved from a long series of BBC radio interviews with the star conducted by John Miller and John Powell. The interviewers and John Goldsmith worked with Sir John in the task of reconstruction, rewriting, and editing.

The resulting account preserves the spontaneity of conversation as it traces and reflects upon Gielgud's extraordinary lifework as actor and director. And the ease and grace of the text will come as no surprise to those who have enjoyed Sir John's previous published works -- "Early Stages," "Stage Directions ," and "Distinguished Company."

For the record, Gielgud retraces the principal forward moves, as well as occasional detours and setbacks, of a career extending over six decades. There are recollections of the famous Terry family, notably great-aunt Ellen, grandmother Kate, uncle Fred, cousin Phyllis Neilson-Terry (who gave him his professional start in 1922), and others. There are glimpses of growing up as the son of a Polish-descended London stockbroker with lively cultural interests. Overcoming a degree of parental reluctance, young John became a scholarship pupil at a small drama school near his grandmother's house and later won a scholarship to the Royal Academy of Dramatic Art.

Gielgud has acted almost all of the great Shakespearean roles (some of them several times) and has directed numerous productions of Shakespeare. On Broadway, his "Hamlet" set a new performance record until that record was surpassed by Richard Burton's "Hamlet," which Gielgud directed.

The challenge to a classics-oriented actor presented by contemporary playwrights is summed up in a revealing passage toward the end of the book. "In Shakespeare," he writes, "I had learned to project, and in Checkhov to act more intimately. . . . Congreve, Wilde and Maugham all demanded somewhat different methods of approach, but I had seemed to achieve some success in interpreting their characters, too. But the plays of Albee, Bond, Storey and Pinter were original and strange, and I relied greatly on Lindsay Anderson and Peter Hall, who directed me so ably in some of these modern plays. . . ."

Theatergoers who have seen David Storey's "Home" and Harold Pinter's "No Man's land" are unlikely to forget the rich performances of Gielgud and Ralph Richardson, his longtime friend and colleague, to whom (along with Lady Richardson) the present memoir is dedicated. The book presents a pantheon of the great players with whom Gielgud has worked, and on whom he appreciatively comments -- Edith Evans ("the finest actress of our time"), Sybil Thorndike, Laurence Olivier, Alec Guinness, and many others in all areas of playmaking.

Parts of "Gielgud" could be grouped under the heading of "advice to the players," wherein the author sets forth some of his accumulated wisdom.

"Gielgud" abounds in actorish stories told with relish and good humor. He is occasionally caustic but never malicious. He is candid and not infrequently self-deprecating. Brecht and Beckett are not among his enthusiasms. And although he enjoys reading Shaw, he has never longed to play any particular part. His major Shavian role was Captain Shotover in the 1977 BBC-TV production of "Heartbreak House." Which incidentally reminds one that his zest for the fresh, new experience explains Gielgud's latter-day success in films and on TV. As he writes in conclusion: "One must go on to the end trying to experiment and break new ground and I am very grateful to have opportunities to do so."

"Gielgud" adds up to a mellow memoir, a book that fascinates, informs, and entertains.