John Updike, often dubbed America's foremost man of letters, is optimistic about the future of the novel both in its respectable and best-seller forms since his works often fit both descriptions.

"Happily it's a large country and English is a large language and there are all sorts of readers out there despite television," said the author of "S.," (Knopf, 279 pp., $17.95), the third novel of a trilogy that harks back to Nathaniel Hawthorne's "The Scarlet Letter."Updike, a likable, soft-spoken man who lives near Boston, wore his gray Yankee tweeds to his publisher's office for an interview about the new novel, his first to have a liberated woman as the heroine.

He said he used to refuse interviews but, "As I have gotten older (he turned 56 March 8), I have gotten more pliable."

"I still won't go on talk shows or cross-country tours," he said. "I don't know if that really sells books or does anything for the novelist's sanity. It certainly doesn't make the book better.

"I'm not a raving best seller, but I'm pleased I sell as well as I do. Certainly my `Roger's Version' had its day in the sun."

He described "Roger's Version," published in 1986, as the core of the trilogy for which the first novel, "A Month of Sundays" and the last, "S.", are the "flankers." These are cerebral novels, taking the three principal characters of "The Scarlet Letter" on odysseys through modern landscapes.

"When I was growing up and going to college, modern authors were being apotheosized," Updike recalled in explaining his writing, which is literary with a strong comic impulse and colloquial flavor. He is generally considered the most perceptive recorder of middle-class manners and mores on the American scene.

"Back then, you were writing to make cultural pronouncements, to solve cultural puzzles," he said. "You were addressing people of like interests and concerns, and literarily respectable authors and the authors of best sellers were not so far apart.

"Steinbeck, for instance, was wholly respectable though very popular. I believe that an author doesn't win readership by catering up or down to his readers, but by writing the best he can.

"Information is what people like, and best sellers James Michener's books for instance seem to be full of information. So are those books about international intrigue and the Cold War that often offer a lot of technical information. Even I have put a glossary in `S.."'

"S." was inspired by what Updike had read about Bhagwan Shree Rajneesh and his Oregon commune. He said there seems to be a market for gurus "because there is an underlying tendency for people to seek for the chance of renewal, to find a new skin that is the underlying message of my book."

Sarah, a feminist who walks out on a confining marriage to seek salvation and regeneration, is one of Updike's most delightful creations: a substantially constructed, unusually verbal woman of earthy appetites, whimsical insights, feminine digressions, unshakable materialism, and not a little wisdom.

"I'd like my next book to be a realistic American novel told in the third person because I've had enough of the first person for a while. It's going to be hard to be realistic about America because so much of life is electronic and fanciful rather than solid pieces of matter. It's hard to be a Dreiser now."

For his next book, Updike has given his publisher a slim volume of six autobiographical essays titled "Self Consciousness."

"The whole book revolves around discomforts, such as dealing with psoriasis and stuttering. It also includes a letter to my two grandsons explaining about being an Updike."