The goal of a lifetime won at last

For Helen Santmyer, success has come somewhat later than it does in most careers. Miss Santmyer, an 88-year-old retired librarian, is being hailed as the literary equivalent of Grandma Moses.

Her novel about small-town life, which she began in the 1920s and finally finished as a nursing home resident in the 1980s, has been published by a university press and is about to be republished in large, lucrative editions by the Book-of-the-Month Club and G.P. Putnam's Sons.

Suddenly Helen Hooven Santmyer is a literary celebrity. Reporters and photographers invade her nursing home accommodations in this quiet southwestern Ohio town. And, as the headlines indicate, the story is of triumph - the goal of a lifetime finally achieved.

Miss Santmyer's 1,344-page novel, '' . . . And Ladies of the Club,'' spans the years 1868-1932 in the small fictional Ohio town of Waynesboro. It follows a group of women across the generations.

Work on it absorbed Miss Santmyer's attention year after year, decade after decade. In the '20s she conceived of the book as a kind of answer to Sinclair Lewis, the tart-tongued novelist who, she felt, had gotten life in small-town America all wrong. She became intent on responding to his ''Main Street,'' which made her so angry that, even decades after its publication in 1920, she seethed when she thought about it. Her friends and acquaintances weren't small, petty people. ''They were strong, independent people,'' she says. ''I wanted to paint a picture of what those men and women did.''

Her own book would tell the true story of small-town life, the life she knew best, the life circumscribed by the courthouse, the church, the school, the railway station, and the cemetery. She spent her childhood years here before going east to Wellesley College, where she graduated in 1918. She published a book and then went abroad in 1924 to Oxford University to study the beginnings of the novel. ''They gave me a bachelor of letters degree,'' she remembers, laughing. ''It was a degree they invented for American students who already had their bachelor's, and they couldn't think of what else to give them.''

She returned to Xenia in the late '20s to care for her mother and father, then served for many years as dean of women and head of the English Department at Cedarville College in southwestern Ohio. Later she worked as a reference librarian in Dayton.

She depicted small-town life in a collection of essays called ''Ohio Town,'' published in 1962. After completing ''Ohio Town,'' she turned back more earnestly to her novel.

''Helen worked on the revision of the book from 1976 on,'' says her friend Mildred Sandoe. And she didn't let ill health and hospitalization stop her. ''She would ask me to bring (the manuscript) out to her, and I would read it to her, and she would make suggestions from the bed,'' Miss Sandoe recalls. In 1982 Ohio State University Press published '' . . . And Ladies of the Club.''

Now it's making even bigger waves. Selection by the Book-of-the-Month Club assures an author of a wide reading audience, and, in this case, a six-figure advance against royalties. ''The standard advance is $85,000,'' says club president Al Silverman, ''but in this case we're paying $110,000.''

And, since G. P. Putnam's Sons has contracted for rights to republish the novel in hard cover in August, sales are likely to be even higher.

Negotiations for adaptation of the book as a TV miniseries or feature film are under way. About the prospect of seeing her book on screen, Miss Santmyer is less than sanguine. ''Movies ruin books,'' she says. ''They take the love affairs and blow everything all out of proportion.''