In a year of theatrical books in which trash has prevailed in the best seller lists, two new volumes celebrating a major star and a less noted player are outstanding.

Katherine Cornell is the subject of "Leading Lady," which Atlantic-Little Brown bills as by "Tad Mosel with Gertrude Macy" ($15.95). Marian seldes titles her Houghton Mifflin autobiography "The Bright Lights" ($9.95).

Both were performers of quality. Cornell was internationally celebrated for over 40 years, 30 of them devoted to her almost-unique career of actress-manager for 39 productions-most of which toured so relentlessly that it has been said that, "Miss Cornell brought back the American road." In "Leading Lady," there are several mentions of the young Seldes, known as a "featured" actress to the public, but respected far beyond that rank within the profession.

Neither volume is of the "let-it-all-hang-out" school, though there are revelations in the Carnell volume that may stun some of her admirers. Both books are devoted to the craft and disciplines of their profession.

Daughter of the late critic and "Omnibus" innovator, Gilbert Seldes, Marian Seldes was completing "The Bright Lights" here last winter while appearing in Arnold Wesker's ill-fated "The Merchant," at the Kennedy Center. In wesker's variation of "Th Merchant of Venice," Seldes had been assigned the new role of Shylock's sister, and Zero Mostel's death, in effect, killed the production. Since then she has been appearing on Broadway in Ira Levin's "Death Trap." She also teaches drama and dance at the Juilliard School.

Seldes' career has never reached the brightest lights. Only rarely has she been "the leading lady", and there have been productions that lasted only a few nights, with her three-season run in "Equus" more the exception than the rule.

But it has been the career of a richly respected survivor, and along the way she has caught the admiration of such associates as Cornell, her husband Guthrie McClintic, Gielgud, Olivier, Burton, Anderson, Gish, Albee, Williams and Robinson Jeffers.

To Seldes, the seriousness of art explains its magnetism:

"How many years of living does it take us to face the Cassandra truth about our lives? Theaters fill up year after year with people searching for answers to the great questions about living and dying. They find in Billie Dawn and Willy Loman and Blanche DuBois, in Oedipus and Hamlet and Saint Joan, the beautiful uniqueness of each human being.

"We go to the theater to find moments that are perfect beyond dreaming-even if they are a part of someone else's life-knowing that in that place of wonder we will be able to relate all that happens to an imaginary character to our own humanness."

For advice, she proffers more than pragmatism:

"Life goes by. There does not seem to be enough time to accomplish in daily living, much less in a career, all that you want to. Your use of time defines the kind of person you are off the stage- and on . . . As Thornton Wilder tells us, 'He who has once been happy is forever out of time's grasp.' "

If Seldes' volume views life subjectively, the collaboration by Mosel and Macy is made objectively. Pulitzer-winning playwright Mosel (for "A Death in the Family") met Cornell only once, as an Air Force private while she was playing in "The Three Sisters." At his request Macy introduced them backstage.

In the summer of 1928, "Gert" Macy, a Californian who had studied at Bryn Mawr and had met Cornell and her husband several times, was tapped to help out Cornell with details of a European trip. She would remain an associate, executive and close friend until Cornell's death, aged 80, in 1974. The book, then, often is Macy's view of the "leading lady."

Further than that, "Leading Lady" serves to mediate between the authobiographies of the McClintics. Miss Cornell wrote her own "as told to" book, "I Wanted To Be an Actress," with Ruth Woodbury Sedgwick in 1938.McClintic wrote his own, "Me and Kit." With the jocular pragmatism that has marked her career as manager and consultant, Macy quotes both versions of the same situations, then gives her own closely observed conclusions.

Beginning as a dazzled admirer, Macy was part of what she now sees as "the institutionalization of Katherine Cornell," a creation of her director-husband, the expectations of such influential friends as Stanton Griffis and A. Conger Good-year, Guthrie's associate Stanley Gilkey, publicist Ray Henderson and Macy herself.

Mosel's construction starts with Cornell's life in Berlin, where she was born, and Buffalo, where she was raised. Guthrie's story begins in Seattle, goes on to New York, and joins Cornell's at the start of their courtship in Detroit where he directed her as a member of the Jessie Bonstelle Company. After a year of making love at parties and in other people's bathrooms, they were married both personally and professionally.

Guthrie would always declare: "I am married to one of the most beautiful voices in the world." That didn't stop him, as the years went by, from male liaisons: "Where Guthrie was indiscreet, he was also secretive and one could not always be sure which of his young male friends were or had been lovers. His affairs seem to have been of fairly short duration, ending without rancor or unpleasantness, the young men often staying on to perform peripheral production jobs."

Later, Mosel writes: "Kit could hardly have lived so many years with Guthrie, sympathetically tolerating his 'other' life, if she had not had a separate emotional life herself. It differed from his in significant ways.

"There was nothing secretive about her emotional attachments. It was always quite evident whom she loved at any given moment. Her letters to her 'bewitching, beguiling' and sometimes 'exasperating' young women friends were typical of the lack of restraint that even to outsiders represented an emotional concentration . . . Unlike Guthrie, she found rapport and spiritual affinity more important than physical gratification."

Those paragraphs, and a few more, are substantially all there is in the 500-page volume on the topic of their sexual preferences. In some biographies such passages would be sensationalized and, thereby, debasing to the work of their lives.

By placing these intimacies in perspective and emphasizing the creative actions of both careers, "Leading Lady" becomes a perceptive, important volume on the two linked careers.

Mosel observes: "As Guthrie brought his art, Gert her probity, and Ray Henderson his aspiration, Nancy Hamilton brought a talent for fun-and has always said she found in Kit one of the funniest women she has ever known."

Later Mosel remarks that "It is to be hoped that Nancy Hamilton, with her perception, wit and gifts as a writer, will one day share her memories of Katharine Cornell. Of all those close to Kit, she was closest after World War II, especially during the last, reflective summing-up years after Guthrie's death and she cannot help but have observations and insights into her friend's entire life that contradict the conjectures and impressions of others.

"It is only by juxtaposing all views that the vastness of Katharine Cornell's nature-and therefore her art-can eventually, effectually be transmitted and understood."

She was a goddess who came to life publicly on hundreds of stages. How much it took-including the willing gifts of countless associates drawn to her-is woven in mesmerizing detail into this study of a life in acting.