The New York Public Library: a storehouse of treasures -- and it's free

One of my favorite New York museums, believe it or not, is the main public library at 5th Avenue and 42nd Street, that massive and slightly forbidding structure guarded by the two marble lions that Fiorello LaGuardia once named Patience and Fortitude.

I learned that last bit of historic trivia on a tour of the library the other day, a switch from my normal role there as researcher and lounger. For some months now I have watched this 1911 Beaux Arts building - one of the five great research libraries in the world - become more and more a showplace and museum, waving colorful banners from the front portals to promote its latest exhibits: photos, prints, paintings dealing with various aspects of bookish history. Of late there has been an elegant display of Melville and Whitman's early New York City, and the lush floral prints of P. J. Redoute, the early 19th-century Belgian botanical illustrator.

There is, then, new life at the New York Public Library, and the man most responsible is Vartan Gregorian, who became president in June 1981. The tireless Mr. Gregorian has just launched the smudged and neglected building on a $20 million restoration program. He even helped pry $5 million from the city, which historically has provided only for the upkeep of the privately endowed library. And Bryant Park, a large green haven behind the library, is on a long overdue cleanup campaign of its own.

Even while the wattage remains too low in the long dim marble corridors and wood file cabinets clutter public spaces, the library remains a house of treasures, and one of the better free tours in town. You can pick up a pamphlet in the main 41st Street entrance and take a self-guided walking tour; or on Monday, Tuesday, and Wednesday at 11 a.m. and 2 p.m. you can tag along for an hour with a library volunteer like Alfred Kronengold and get the inside story.

Mr. Kronengold gathered a dozen of us in a corner of the vaulting Astor Hall and dispensed with the statistics: 5 million volumes, 11 million manuscripts, 90 miles of shelves. Almost all of this is available to the public, not the case at the more cloistered, private British Library, Bibliotheque Nationale in Paris, Moscow's Lenin State Library, and the US Library of Congress, the four other great research libraries. You can't check a book out of the 42nd Street library, but there must be 90 miles of tables and chairs at which to enjoy the contents.

Outside the Map Room (117), Mr. Kronengold told us rather conspiratorially, ''There are captured World War II German maps in there, maps the US Army and Navy studied before invasions, even some maps made of bread so the troops could eat the evidence if necessary.''

We ascended to the third floor in an elevator, passing up the soaring marble stairways I use for exercise on sedentary research days, and paused outside the A. A. Berg Collection (Room 320), a valuable English and American literature cache. There are first editions, correspondences between authors and publishers, items such as T. S. Eliot's typescript of ''The Waste Land'' with Ezra Pound's scrawled corrections. This room and several others are generally set aside for scholars, and yet anyone with a reasonable request may gain entry by applying to Room 226.

Part of the Berg Collection, in Room 318, is totally public, and here we scanned the latest in a revolving series of literary exhibits - letters, manuscripts, and bits of verse by the Irish poet and novelist James Stephens on the occasion of his birthday centennial. On the walls are portraits of George Washington by Peale and Gilbert Stuart and one of Lafayette by Samuel F. B. Morse.

Down the hall we passed under colorful WPA murals depicting great moments in publishing (the Gutenberg Bible being printed, Mergenthaler at his linotype machine) and entered Room 315, a familiar destination. Here in the catalog room begins that magic hunt for one of the 5 million volumes. It can be daunting or, once mastered, as heady as a child's electronic game.

As Mr. Kronengold explained it, you first look up the call number and hand the slip to a clerk who sends it off by pneumatic tube and gives you a numbered ticket. You pass into the vast north and south reading rooms and wait for the green electric board to light up with your number, signifying that the book has been tracked down by a page on eight hidden flights of stacks below and delivered by dumbwaiter. Around you on the open shelves are 30,000 reference books - encyclopedias, biographies, European and American phone books. At the back of the north hall is the Genealogical Division with its parish registers and 19th-century ship-landing lists - a favorite haunt of roots-tracers.

Perhaps the reading room lacks the romance of the British Museum where Karl Marx wrote ''Das Kapital,'' but did you know that the American Dream has been achieved here more than once? Edwin Land did the research that resulted in the Polaroid Land Camera, and Juan Trippe, the Pan Am founder, ''discovered'' Wake Island in old clipper ship logs and opened the Pacific to commercial air travel.

Our last stop was outside the Frederick Lewis Allen Room, one of New York's most exclusive clubs. There are 16 carrels set aside for writers with publishing contracts; and a long waiting list. Betty Friedan, Theodore White, and Nancy Milford wrote books in these nooks.

There are plans under president Gregorian to extend this service and also to restore certain out-of-use rooms, to bathe the exterior in floodlights, and to give the two marble lions a washing. And one of these days, if the money holds out, the library's hours may be extended to the seven-day schedule it kept until the early 1970s (it is closed now on Thursday and Sunday). For scholars, library loungers, and museum lovers, this is the most treasured news of all.