SOME OF THE MAJOR ink users of the western world these days are squirreled away inside those brick and marble heaps around the Mall. The Department of Agriculture alone accounts for 3,000 publications and people write in for them at the rate of almost 200,000 a year. They range from Removing Stains from Fabrics to Solar Energy for Milking Parlors. Some are in Spanish; some are on microfiche, and "List Eleven," which lists all the titles on hand, is a book 185 pages long.

The Agriculture Yearbook , which first came out in 1894 and has appeared in its present form since 1936, is the flagship of the fleet. Trees , the issue for 1949, and COnsumers All , in 1965, had sales of more than 120,000 apiece. Gardening for Food and Fun , this year's Yearbook , has just come and costs $6.50 - up form $1.25 in 1936.

The Smithsonian Institution has been exuberantly into publishing since 1848, when it put up out Ancient Monuments of the Mississippi Valley , a massive tone with spidery engravings and elaborate maps, written by E. G. Squier, A. M. and E. H. Davis, M.D. In 1966, after 118 years of other works in this vein, Smithsonian publishing was reborn as the Smithsonian Institution Press, which joined the American Association of University Presses and strode boldly off in new directions, "The Press now wears two hats," says Felix Lowe, its deputy director.

Under Hat No. 1, it produces the floor plans, catalogues, folders and other materials that accompany the exhibits and festivals that go on over there.It also puts out Federally funded books that are sold and distributed through the Superintendent of Documents (or "Soup Docs," as Mr. Lowe calls him). A recent "Federal book" was entitled The Systematic, Postmarsupial Development, and Ecology of the Deep-Sea Family Neotanaidae (Crustacea: Tanaidacea) by Lion F. Gardiner, and Soup Docs distributed it to a carefully screened list of 2,000 scholars, special libraries and such.

Under Hat No. 2, the "private side," the Smithsonian Press behaves like a regular workaday publisher - editing manuscrupts, designing the books and having them printed. They distribute the private books through the field sales netwoek of Columbia University Sales Consortium, and by direct mail. They're on sale here at places like Kramer's and Discount Books and in the museum stores. The Zoo Book ($3.95 paperback; $8.95 hard cover) won several awards and sold 20,000 copies last year. Flora of Okinawa and the Southern Ryukyu Islands by Egbert H. Walker sold 500 of the 1,278 copies printed - at $36.75 - the year it came out. The Smithsonian Collection of Newspaper Comics , edited by Bill Blackbeard and Martin Williams, co-published witha Harry N. Abrams Inc., the New York art book publisher, and printed in Japan, comes out next February and is already a Book-of-the-Month Club alternate selection. "Our goal," Felix Lowe says, "is to break even with these private books in the next three to five years."

Over in the Postal Service Building on L'Enfant Plaza, the Smithsonian Publishing Task Force, headed by Jake page, an alumnus of Smithsonian Magazine, has been looking into whether the Smithsonian could "legitimately and reasonably, without inviting disaster, do some general interest publishing."

They decided that it could, and the first experiment, The Smithsonian Exprience , a big handsome volume, loaded with everything from paintings, photos and drawings to a genuine Ecuadoran recipe for shrinking humans heads, has already "had a staggering response," Page says, with 130,000 orders before it even appeared. It sells for $14.97 to Smithsonian Associates and will be $19.95 in bookstores, distributed by W. W. Norton. So the Task Force is back testing some more ideas for books to tap the fathomless riches in the nooks and crannies of the "nation's attic."

Over in the National Gallery, the Publications Program has been putting out catalogues, books, post cards and posters since the gallery opened in 1941. Its best sellers tend to be the brochures and books connected with its block-buster exhibits and its peculiarity is that the program pay its own way. The books that make a profit help carry the weight of the one that don't. Happy Endings

NOTHING INTERFERES with your day - if you'r trying to write a book - like a full-time job. Nathan Miller turned out Sea of Glory , about the U.S. Navy in th Revolution and The Founding Finaglers , a history of corruption from Jamestown to Watergate while working full-time for the Senate Appropriations Committee. But this October, with The U.S. Navy: An Illustrated History , co-published by the Naval Institute Press and American Heritage, with bookstore sales by Simon and Schuster, he hit the jackpot. It sold more than 35,000 copies before it was published, was a Military Book Club selection and a Literary Guild alternate and chosen as a textbook by the U.S. Naval Academy. Now, with a contract from Doubleday for a book on the Roosevelts and another from the Naval Institute for the history of Naval aviation, he's quit his full-time job and settled at home as a full-time author - not counting the two months he's spending as temporary editor of the Washington Journalism Review. And as if that weren't enough - he was the last person Barbara Walters interviewed live on the Today show.