Folger Shakespeare Library

The dusters, it is said, begin their work at one end of the stacks of bound leather books and, volume by volume (''Titus,'' 1594, by ''Shakespeare's Plays, '' 1685, one assumes), make their way back to the start . . . to dust again and again . . . until infinity wears the pages thin.

So the days, weeks, months, must go at the Folger Shakespeare Library here.

From 1891 to 1929, Henry Clay Folger and Emily Jordan Folger also made their way - to England, to American dealers, anywhere - to amass these volumes (11 summer trips to England alone). ''Single-mindedly,'' the oil magnate and his wife secured a quantity of Shakespeare books second only to Great Britain's.

It is also said that the 3,000 cases in storage took six months to transport to the home the Folgers built for themselves in Washington in 1932. A literary palace outfitted in a marble exterior and fitted out with a mix of Elizabethan and Art Moderne elements arose: Architect Paul Philippe Cret had created one of the most lavish book boxes in America.

No wonder a reverence for things past characterized the attitude toward this 50-year-old repository of 93,000 books, 50,000 prints and engravings, and thousands of manuscripts. No wonder, too, the prospect of providing more space for the scholars who use them intimidated the would-be builders. And no wonder, in the end, a building that was a parallel to Cret's careful architectural extravaganza emerged.

The neo-neo-classical or post-modern or historicist or - pick a name - $8 million addition that opened this winter is an often marvelous makeover and expansion by Washington architects Hartman-Cox.

Far from assaulting the visitor with this hodgepodge, the design is a nip-and-tuck architecture. It updates without overwhelming the past, and adjusts without bowing and scraping to the Cret original.

Slotting their new space into the old ''U'' of the Folger, the firm inched along in an eight-year effort (headed by Warren Hartman with Mario Boriandi). Their addition of reading room and stack space, plus altered offices, is neither subservient nor superior - a rare achievement.

Adding to an old and venerated building is an awesome task. And if the drift in design is to do just this (more than half of the new work will be on old buildings, the American Institute of Architects recently estimated), it is an essential one.

On the one hand, a ''don't tread on me'' attitude to an old building may produce mediocrity; on the other, an urge for creative excellence to match a masterpiece may vanquish or leave it breathless.

One famous instance, adding to Eero Saarinen's magnificent John Deere plant of 1964 in Moline, Ill., resulted, for all architect Kevin Roche's labors, in a 200,000-square-foot addition whose material overabundance underscores the genius in-simplicity of the original.

Perhaps because the Folger's architects reside in more modest ranks or perhaps because worship for a Cret does not tax the creative spirit to the same degree, the addition to the Folger Shakespeare Library is a a symbol of the art of nice fit.

The work, ranging from the very visible to the almost invisible, is not a total success. The marble addition to the rear of the building mixes steel rods and projecting marble columns somewhat awkwardly; the dark glass lining the top of the Georgia marble facade does not quite mesh.

Cret's 1930 exterior was a ponderous if elegant tomb designed to match nearby government structures like the Library of Congress, but the Capitol Hill classicism of his exterior was relieved by the pattern of aluminum grillwork and carvings. The contrast was pronounced with the library's pageant of chambers, the Tudor half-beam-style stage sets within.

Nonetheless, the Hartman-Cox interior addition is excellent. It is inconspicuous where need be, in the air conditioning and dehumidifying apparatus as well as in the addition of new oak shelving or panels switched from the old quarters to the new. And it is bolder on occasion.

The Washington firm had both a technological problem and an aesthetic one. ''It's like a submarine,'' says Warren Cox; ''everything had to fit.'' The firm succeeded on both scores.

The reading room is the showpiece of the Cox-Hartman amenities. It is a fine blend of modernist urges for skylights and lofty windows, and historicist shaping and shadowing of space through vaulted ceilings and apselike smaller spaces at either end. The new reading room replays the form of the adjacent, richly embellished old reading room but adds a contrast to its cloistered atmosphere.

At times the then-or-now element - which is 1930s, which 1980s? - in the renovation wears. The revivalist furniture in the reading room is hard-grained and appropriate, but might have wandered further afield from period reproduction. ''This is new, this is old,'' punctuates Cox's tour, culminating in the rusticated arch done in wood in the stacks. And sometimes it would have served better to make the age visually clearer.

All this works well, well enough to earn awards for the plasterers and a prize for millwork. And yet the sudden appearance of a lavish vault, glistening like some stunning medieval/modern toy, makes you wonder if a bit more pizazz might have relieved some ponderousness.

Reverence is worthwhile, but a Shakespeare collection - any collection, really - is not only a rare hoarding but an obsession: a divine obsession but also one that is devoted to trivia. It might benefit from a light touch.

The art on Shakespeare around the library offers, for instance, the languid ladies and leggy lords from Shakespeare's plays. The playwright's portraits or a candy-box Juliet are close to kitsch. Is it too much to miss some of that inspired silliness in the design that frames them?

Nevertheless, the library looks and works well. Hartman-Cox has created an addition at ease with the old building's Elizabethan inspiration and its eccentric architecture. And it is a graceful gift from America to the Bard who first uttered ''O Brave New World.''