YOU MAY have noticed in your local bookstore a section devoted to large-print books. Though sales of individual titles remain small -- 4,000 is a best seller for a large-print book -- more and more such titles are appearing every month and there is now a book club for readers of large-print books.

The guru of the large-print book in the United States is a third-generation San Franciscan named Lorraine Marchi, who is founder and head of the National Association for the Visually Handicapped (NAVH). Her interest in the subject began in the early 1950s when she discovered that there was no reading material available for her son who had just entered the first grade. Though not legally blind, he did have serious vision impairment.

"There are 498,000 legally blind people, and they are served by 800 agencies," says Lorraine Marchi. "But there are close to 11 million people who are visually handicapped, which means that even with the best correction, the vision in their better eye is less than normal. Among other things, that means in most states the denial of a driver's license on grounds of vision." It was for this group, and for the 90 percent of legally blind people who had some usable vision, that Mrs. Marchi set up the NAVH.

Among the organization's first aims was to make school-related textbooks available in large print for the visually handicapped. The first four large-print books it published in San Francisco in the 1950s included a fifth-grade speller and three social studies books for the third grade. The NAVH now has a backlist of about 200 books. In the 1960s, an English firm called Ulverscroft began to do large-type editions for adults and a few years later Franklin Watts followed suit here. In the late 1960s, Mrs. Marchi persuaded major publishers including Harper & Row, Simon and Schuster, Prentice-Hall, Scribners and Walker & Co. to give large-print publishing for adults a try, but all later abandoned it because, she says, the small press runs made the books money losers. Walker & Co. has since returned to the large-print market.

The concept of "large print" has different interpretations, but the NAVH has a list of standards that make 16-point type a desideratum. A point equals 1/72 of an inch. Normal book type generally runs from 9 to 11 points, and Book Report is printed in 8.5 point type. "But," says Mrs. Marchi, "point size is only one factor. The amount of white space between lines, the blackness of the type, and the color of the paper are among other important considerations."

Though large-print books are only a small part of its overall business, the biggest name in the field is G.K. Hall, a Boston publisher that is strong in the library market. Probably its best-known division is Twayne, which does critical and biographical works aimed at college students. Hall began to do large-print books in 1970 -- buying rights from other publishers -- and now issues 200 titles a year, primarily fiction. According to Janice Meagher, who has been Hall's executive editor for large-print titles since Jan. 1, the company's best-selling title in the area is probably the Merriam-Webster Dictionary for Large-Print Users, first issued in 1975. Hall also does well with The Fanny Farmer Cookbook and with a series of original anthologies including Best-Loved Poems in Large Print. Reflecting the fact that readers of large-print books tend to be older people, another best-selling book on the Hall list is Sex After Sixty. THE OTHER TWO major publishers in the large-print field are both run by people who once worked for G.K. Hall -- Phillips Treleaven, who was president of Hall and now heads the Thorndike Publishing Company, and John Curley, a former marketing man with Hall who is boss of John Curley & Associates.

Thorndike is located in the town of Thorndike, Maine, which has a population of 500 -- "more cows than people," as Phillips Treleaven describes it. The company publishes 88 titles a year, plus 36 romances imported from the British arm of Harlequin Books, Mills & Boone. Its best-seller so far has been Lake Wobegon Days by Garrison Keillor, but it also has high hopes for its recently published edition of Jean Auel's The Mammoth Hunters. Because of the larger type size involved, the book was published in two volumes.

In addition to its large-print line, Thorndike also has done about 30 books of local interest in regular type sizes. How to Talk Yankee, a humorous book by schoolteacher Gerald E. Lewis on the language of New England, has sold 90,000 copies for Thorndike.

A lover of Cape Cod, John Curley used to commute into Boston from South Yarmouth, Massachusetts, when he worked for Hall. Now he runs his business from there and can go out for a swim or a game of golf whenever he wants. Curley does nothing but large-print books. The company does about 250 titles a year, including two romances, two mysteries and two westerns every month. Starting next January, the number of westerns will rise to three a month. According to John Curley's daughter, Mary, who works for the company, its best-selling author is probably Zane Grey. Titles on Curley's upcoming fall list include The Patient Has the Floor by Alistair Cooke, Tefuga by Peter Dickinson, John Hersey's Hiroshima and biographies of John Wayne and Ethel Merman.

Walker & Co., a mainstream New York publisher, concentrates on "inspirational classics" in its large-print line, including The Prophet by Khalil Gibran, Jonathan Livingston Seagull by Richard Bach, Strength to Love by Martin Luther King and The Power of Positive Thinking by Norman Vincent Peale. Walker has 67 large-print books in its backlist.

A new entry in the large-print sweepstakes is Isis, a company based in Oxford, England. Its American marketing is handled out of Brooklyn, N.Y., by Allen Kleiman, a librarian specializing in services for the aging, and his wife Rhonda. Isis has done about 50 books so far, issuing three nonfiction titles and two fiction each month. Its best seller so far has been Dr. Christiaan Barnard's Arthritis Handbook and in the fall it will be publishing Barnard's Your Healthy Heart.

AN IMPORTANT development in the large-print field is the establishment -- as part of the Doubleday group of book clubs (which includes the Literary Guild) -- of the Doubleday Large Print Home Library. The club gets its selections from books under contract to its fellow clubs in the Doubleday group. Current offerings include I'll Take Manhattan by Judith Krantz, High Jinx by William F. Buckley Jr., Break In by Dick Francis and Murder at the FBI by Margaret Truman. For information, write to the club in Garden City, N.Y. 11534-1104. The club also has an 800 number: 800-343-4300, ext. 355.

Another source for users of large-print books is the library maintained by the NAVH, 22 West 21st Street, New York 10010 (telephone 212-889-3141). Up to two large-print books can be checked out of the library at any given time and mailed anywere in the country. The NAVH also provides vision information, book catalogs and a free newsletter.

The latest development in the large-print field harks back to the beginnings of the NAVH and Mrs. Marchi's frustration at not being able to obtain books for her son. G.K. Hall has begun to publish children's books in large type, including such classics as Five on a Treasure Island by Enid Blyton, Mrs. Frisby and the Rats of NIMH by Robert C. O'Brien and Are You There, God? It's Me, Margaret by Judy Blume. And they are excited by their seven-volume edition of C.S. Lewis's The Chronicles of Narnia, due in the fall.

And a sign of the times. While marketing people at the large-print companies see their main audience as the growing population of older people, they think there may be a secondary audience in another group: younger people who work in front of computer screens all day and come home with very tired eyes.