EVERYBODY'S doing it now, recording the finest offerings of American popular music from the catalogues of classic songwriters. Harold Arlen. Or Jerome Kern. Yip Harburg or Cole Porter.

For Ella Fitzgerald, the First Lady of Song who kicks off the second Kool Jazz Festival at the Kennedy Center on Saturday, this must seem old hat.

She recorded the first of her fabled "songbooks" in 1956, and in a three-year span immortalized the repertoires of Rodgers and Hart, Cole Porter, George and Ira Gershwin, Jerome Kern, Irving Berlin and Duke Ellington. These weren't simple projects either.

"Ella Fitzgerald Sings the George and Ira Gershwin Song Book" (Verve import 2615 063), the latest reissue, is a five-volume set containing 53 selections. The Ellington project, spread out over two double sets, contains 38 selections. "Volume Two: The Small Group Sessions" (Verve VE2-2540) was also recently reissued, joining the Porter, Rodgers & Hart, the first Ellington volume and an abbreviated Gershwin set, all of which have been put back into circulation since 1977.

Few singers ever have been better equipped to handle such a massive undertaking and the remarkable thing is that Fitzgerald sounds as good and as perennially ebullient at age 72 as she did in the mid-'50s and in the late '30s when she fronted the Chick Webb Orchestra. The '50s marked a flowering for Fitzgerald, particularly her severance from Decca Records, which had weighed her down with overproduction and novelty numbers in the hopes of repeating the smashing success of "A Tisket a Tasket."

Fitzgerald's qualities have been so consistent and familiar over the years that she's sometimes taken for granted (remember, she started out at the same time as Billie Holiday). Her voice exhibits a rhythmically vigorous, yet extremely warm timbre, an inspiring simplicity and unabashed enthusiasm, as if she was drinking from the fountain of youthfulness. Though her voice has over the years evolved onto a deep contralto, Fitzgerald still zooms up and down the scales in pure, perfect pitch, a vocal butterfly chasing a melody line.

There are two distinct sides to her art: on the one hand, Fitzgerald may be the greatest popular singer of our time, but in the last 25 years she's balanced that with her "jazz singer" personality--a swirl of vocal improvisations, driving scats and, always, that impeccable and distinctly personal sense of time that marks the great jazz artist. As one critic pointed out, in balancing fine control with vivid improvisation, Fitzgerald has managed to be both a singer's singer and the public's singer.

The songbooks, with the possible exception of the Ellington volumes, address Fitzgerald more as the popular singer than as the jazz artist. Although her interpretations--generally straight ahead and devoid of noticeable embellishment--haven't dated, the orchestrations (by Nelson Riddle on the Gershwin, by Buddy Bregman on the Porter and Rodgers and Hart) certainly have. Riddle's work, most often either a swarm of brassy accents or a swirl of strings and woodwinds, may have been commercially sound at the time, but its rapidly exposed sameness only reinforces the timelessness of Fitzgerald's contribution.

Fitzgerald remains one of the most literate singers in her ability to understand and convey the inner meaning of a lyric, often imparting a subtle personal intepretation that suggests a longtime familiarity with a song. She manages this even within the context of a straightforward setting, though even she can't save the immense numbers of turkeys on the Gershwin set. That's the danger of such a large project, and one is thankful Fitzgerald didn't have to dip quite so deeply into Kern or Rodgers and Hart. I mean, the Gershwins' "Just Another Rhumba" does manage to rhyme "numbah, dumbah, succumbah [and] slumba," but it's just not a song that's remained on the lips of America. Come to think of it, the Gershwin set confirms the erratic output of the brothers: for every "But Not for Me" and "Embraceable You," there's a "Sam and Delilah" and "The Real American Folk Song."

For the most part, though, Fitzgerald has chosen chestnuts that have been roasting on the fire of popular music for decades. These are the blue-chip standards that have entranced the singers and the jazz improvisers, the listeners and the cocktail lounge pianists since escaping from long-forgotten Broadway shows and Hollywood film fantasies into the American psyche.

In the Gershwin set, she performs with the same qualities that endeared her to Richard Rodgers, investing the lyrics with a grace and elegance he defined as "intimacy, naturalness and realism--she portrays the songs, she does not betray them." For instance, a soft-sung "But Not for Me" is achingly melancholic, with Fitzgerald plumbing the lower emotional depths. On "Let's Call the Whole Thing Off," she's playful yet insistent. "Lady Be Good" is languid, lyrical and liquid, a modern lullaby. "Nice Work If You Can Get It" is finessed into a soft swinging gait, while "S'Wonderful" is appropriately radiant and gleeful.

And so it goes, for 10 sides: a pure-hearted "Someone to Watch Over Me," the triumphant "Who's got the last laugh now" of "They All Laughed," the sensually inviting "Shall We Dance," the plaintive hope for "Somebody From Somewhere (For Nobody but Me)." There are few Gershwin favorites left out (though there's nothing from "Porgy and Bess," a Fitzgerald-Louis Armstrong version of which was reissued on Verve several years ago). These are not necessarily the definitive interpretations of the songs, though many have remained in the Fitzgerald repertoire for decades. They are, however, consistently fine, a library to be dipped into again and again.

On the first volume of the "Duke Ellington Songbook," Ellington wrote a four-part "Portrait of Ella Fitzgerald." It was not one of his better works, but the names of the segments gave a pretty good idea of how he felt about her: "Royal Ancestry," "All Heart," "Beyond Category" and "Total Jazz."

On "Volume One" (Verve VE2-2535), Fitzgerald tended to be a bit overwhelmed by the Ellington band. However, the sextet and quintet settings of "Volume Two" were a perfect compromise between the intimacy of her favored trios and unwieldly orchestras. And the lean, unhurried, yet still enveloping settings by the small Ellington ensemble were certainly less saccharine than the syrupy backing provided for the other songbooks. The result is a personal involvement in the material, even though Fitzgerald was technically an outsider. More of her personality comes through, particularly on the ballads, where she shared the focus with the wonderful Ben Webster and, just as often, the seminal black violinist, Stuff Smith.

What's interesting here is that many of the tunes--in fact, 15 of the 19 on the album--originally were conceived as instrumentals. The addition of words would alter some of them, but some never got "real" words until well after this recording (like the sensual "Satin Doll," which may be basically wordless but is surprisingly eloquent).

On "Sophisticated Lady," Fitzgerald softly bends the melody line before Smith launches into a vibrant solo. After that comes the first of many marvelous and mellifluous Webster solos. Webster was at the height of his powers when these sessions took place in 1956 and Ellington's sinewy melody lines, particularly in the ballads, seemed to bring out the best in him.

For instance on "Do Nothing Till You Hear From Me," there's a surety to his supple embellishments, a pure grace in his trademark breathy vibrato. Webster's rich, agile tone was a perfect complement to Fitzgerald's voice, as "In a Mellotone" proves: On this classic, Fitzgerald is just one of the boys, taking her break with a voice instead of an instrument.

There's something foreveryone here, from three subtle duets with guitarist Kenny Burrell ("Solitude," "In a Sentimental Mood" and "Azure") to some assured scatting on "It Don't Mean a Thing," "Cottontail" and "Squatty Roo." Fitzgerald sashays through the implied lament of "Don't Get Around Much Anymore," and shows a rare bluesy edge in the languid, world-weary "Rocks in My Bed."

As her trios have been proving for years, Fitzgerald needs only minimal accompaniment, but these sessions provided an earthiness and a subtle urgency sometimes lacking in the other songbook projects. One feels Fitzgerald got to sing the Ellington songs in her own time, that she was able to be a little less respectful in her attention to the text.

After all these years, she has maintained the joyous spirit and perfect intonation that have always marked her work. One fan said "Ella's heart sings through her voice." It's a big, big heart.