Its zenith may have been 40 years ago, but melodies from the golden age of American Popular Song linger on. Currently championed in the spotlight by pop stars Linda Ronstadt and Barry Manilow, subtly sustained by scores of cabaret and jazz singers who have never abandoned the elegant and eloquent missives of the Gershwins, Cole Porter, Jerome Kern, Irving Berlin and others, American Popular Song demands attention.

Ronstadt's appreciation for -- or perhaps appropriation of -- the music that dominated this country for more than half a century is put to the test on her new album. Showing that opportunism does indeed knock more than once, Ronstadt has gone to the well again on "Lush Life" (Asylum 9 60387-1), a collection of a dozen standards, or, as she would probably insist, sophisticated songs.

Manilow has taken a different approach, gathering an all-star jazz ensemble for an "in the wee small hours" type album that owes everything but the songs to Frank Sinatra. The songs come from Manilow himself and while he's no Harold Arlen, he's at least craftsman enough to slip into the style comfortably and without pretension.

Ironically, the set that is most rewarding and revealing comes not from pop institutions like Ronstadt and Manilow, but from ye old Smithsonian, which has just released a magnificent collection called "American Popular Song," 110 of them to be precise. Four of the songs on "Lush Life" appear in the Smithsonian collection, and anyone who wants to observe the difference between polyester and silk is advised to shop and compare.

Come to think of it, when Ronstadt leaves "La Bohe'me," she should retire to her stereo and immerse herself in "American Popular Song: Six Decades of Songwriters and Singers," a brilliant new collection (seven albums or four cassettes, plus a 152-page book) from the Smithsonian Collection of Recordings. Like the jazz and country collections before it, this is a landmark work of immense musical and historical value.

"American Popular Song" celebrates the flowering of song that occurred roughly between the mid 1900s and the mid-'50s, a period as distinct and visceral as the 19th-century German lieder and French chanson movements. National characteristics -- verbal, rhythmic, melodic and harmonic -- would play a central and enduring role, but the sheer quantity and quality of the songs themselves would be self-defining. As the book says, "during these years a group of composers and lyricists produced songs that were so well-crafted in their melodic episodes, so sensitive and responsive in their word settings that they stand as a distinctive achievement in our culture."

The Smithsonian collection is culled from more than 6,000 versions of an original working group of 500 songs. Although there are many composers, lyricists and singers involved, "American Popular Song" has its dominant personalities. On the song side, they include Berlin, Kern, the Gershwins, Porter, Richard Rodgers, Lorenz Hart, Dorothy Fields, Howard Dietz, Arthur Schwartz, Johnny Mercer and Duke Ellington. The most frequently heard of 62 performers are Marian Harris (a revelation), Fred Astaire, Bing Crosby, Helen Forrest, Judy Garland, Lena Horne, Nat (King) Cole, Frank Sinatra, Sarah Vaughan, Ella Fitzgerald, Carmen McRae, Mel Torme and Tony Bennett.

Whew.

That's some hall of fame and the material is equally resplendent. There isn't that much gold under Fort Knox and even if there were, it's doubtful it would be mined as resourcefully or shaped as imaginatively as this set has been by the Smithsonian's James R. Morris, J.R. Taylor and Dwight B. Bowers.

In the accompanying book, they have reached back to the 12th century to define the roots of song itself; observed the changes wrought by developing technology of radio, recordings, film and television; analyzed six decades worth of transformations within the idiom; restated the contributions of black culture all the way back to Stephen Foster and minstrel shows; shown how popular song mirrored advances in literature, architecture and art, as well as changes in language and social values; examined the parallel progressions in writing and singing.

In presenting the inextricable linkage of events, Morris, Taylor and Bowers illuminate without ever becoming overbearing. They have designed the set chronologically, with the emphasis on the songs rather than the singers, the style rather than the stylists. That's why you will hear few readings by jazz singers, whose brilliance and invention were often at odds with the songwriters' intentions (though they could just as often elevate the material to a higher level). Jazz musicians, of course, return to the well time and time again, inspired by the genre's intelligent melodies, but the versions here are pretty straightforward.

This is not to suggest that there isn't a plethora of intensely personal styles, and an abundance of structures, on this collection. It's just subsumed to celebrate the blossoming of an intensely American art form. And lest all this sound dry, it's not. It's absolutely enthralling, from the scratchy sound of a Sophie Tucker 78 ("Some of These Days," recorded in 1911), Al Jolson's declamatory style on "April Showers" and Grace Kerns and Reed Miller's "They Didn't Believe Me" (a 1915 recording that marks the diminishing European influence) to Barbara Cook's 1980 recording of "Some Other Time."

Here's another taste -- seven songs that ask some universal questions:

"Why Was I Born?"

"Brother, Can You Spare a Dime?"

"Isn't This a Lovely Day?"

"What Is This Thing Called Love?"

"How Long Has This Been Going On?"

"Isn't It Romantic?"

"What'll I Do?"

What else is there to know?

The process of each song is briefly analyzed (as are the songwriters and singers) and if it's astounding enough simply to listen to these songs, it's more so when you relisten with context. The commentary is illustrative and revealing about the mannerisms encasing the songs between creation and delivery, but it's the songs, not the text, that you'll be going back to time and again.

There is no better place to be. That's something that the singers discovered, as well. It's interesting to note that half the material on "Popular American Song" was recorded in the '50s (paralleling the advent of long-playing records) but an equally large percentage of the songs were written during the '30s, suggesting that the movement had peaked even then. It's an idea reinforced by the fact that after 1940, the gap between the writing of a song and its performance widened. When the rails of material and performance no longer ran parallel, the nails of rock and roll, rhythm and blues and country music were being held to the coffin, driven in by a post-war generation ready for some major changes.

One can't review this collection; one can only be thankful for it. "American Popular Song" defines an era, celebrates many of its most glorious moments. Despite the occasional Stephen Sondheim, it is an era that has passed, a movement that exists in a million memories and on thousands of recordings. Because of its nonprofit, educational status, the Smithsonian has been able to build its three landmark collections by drawing material from many different record companies and from collectors. Feel free to applaud between songs.

There are many who have applauded Linda Ronstadt for the courageous career moves that have taken her away from her natural pop inclinations and into light opera and torch songs. Courage doesn't necessarily provoke art, however, and "Lush Life" shows little gain over last year's "What's New?" It's not a question of dues-paying, or even homage-paying, or the fact that there are literally dozens of singers better suited to the material struggling for survival in an environment that for the most part turned its back on them almost 30 years ago. Ronstadt may indeed care deeply for this material, and for those who originally defined it. But she does not inform it with anything new and her shortcomings are serious.

The most serious is her lack of interpretive skill. Most of the songs on "Lush Life" are intensely personal, dominated with the first person singular ("I'm a Fool to Want You," "When I Fall in Love," "It Never Entered My Mind," "My Old Flame"). "I" and "my" dominate, yet there is nothing personal about Ronstadt's readings. She never seems to plumb the lyrics, to get inside the songs, and of course, that's the trick and the rub. It won't do to simply run them through, the voice as saber.

Although three songs provide up-tempo relief from the sameness that sank "What's New," "Lush Life" sounds a single piece. Ronstadt's phrasing is both precise and uncertain, head-centered rather than heart-powered. Her rhythmic instincts are dull; when she tries to swing "Falling in Love Again," Ronstadt simply falls. Her dynamics approximate gear-shifting and she seldom slides gracefully into any modulations. Her vocals are monochrome, sometimes strident and she is simply inept and too well-mannered to be convincing. If only her covers were as good as her album covers ("Lush Life" approximates an old-fashioned hat box: pop it open and Here's Linda!).

Once again, Ronstadt has purchased the protective coloring of the Nelson Riddle Orchestra. You will not, in fact, find such lush orchestrations anywhere else in today's music outside of Sinatra; no one else can afford them. Riddle, now more familiar with Ronstadt's limitations, acquits himself better this time around, particularly on the promising sass of "You Took Advantage of Me." Once again, the glossy arrangements are punctuated by sharp solo spots and Riddle does his best to provoke the singer out of her precision, to no particular avail.

If Ronstadt's "What's New" was a bow to Frank Sinatra's "Only the Lonely," then Barry Manilow's "2 a.m. Paradise Cafe" (Arista AL8-8254) is homage to Sinatra's "In the Wee Small Hours." By crafting his own songs, of course, Manilow avoids direct comparison. And he's wise to allow gilt by association: he surrounds his own supple piano with an all-star combo, including baritone saxophonist Gerry Mulligan (who gives the music weight), elegant bassist George Duvivier and guitarist Mundell Lowe (who give it class) and the late Shelly Manne on drums (he provided the sinewy pulse). Sarah Vaughan and Mel Torme contribute rather perfunctory duets, but the album is mostly Manilow.

And Manilow, who's been a gifted formula writer since his days in the jingle business, knows his territory. If you didn't know any better, you might think "What Am I Doing Here" and "Blue" and the title cut are more than a year old; Manilow's melodies are familiar enough. But the lyrics, mostly by Marty Panzer, Bruce Sussman, Jack Feldman or Adrienne Anderson are dull-witted and derivative. Even the Johnny Mercer lyric on "When October Goes" is substandard. The singing is warm, straightforward and seldom overeaches. This is a genial appreciation.

Tasty arrangements and appropriate mood do not a great album make, however. At least Manilow's work lacks the high culture pretension that so informs Ronstadt's recent activities. Besides the Smithsonian collection, she might turn to the spate of "songbook" albums that have become popular in the last few years. Mostly reissues of '50s recordings by the likes of Ella Fitzgerald and Sarah Vaughan, these don't just use the standards, they set them.

PolyGram Classics, which has already reissued a slew of Fitzgerald songbooks, has just instituted an appropriately-titled new series, Great American Songbooks, on its Verve and Emarcy labels. The indefatigable Fitzgerald is represented on "The Johnny Mercer Songbook" (Verve 823 247-1), a 1952 session that finds her in the company of . . . the Nelson Riddle Orchestra!

Manilow mates Vaughan and Torme are also represented, with Torme racing through "The Duke Ellington and Count Basie Songbook" (Verve 823 248-2) and Vaughan joining Billy Eckstine on "The Irving Berlin Songbook" (Emarcy 822 526-1). The Torme dates from 1961, the Vaughan-Eckstine from 1955. The initial release is completed by Dinah Washington's "The Fats Waller Songbook" (Emarcy 818 930-1), from 1958.

Like the songs themselves, class will tell over time. It's not that Ronstadt is the only young champion, either. If you want to hear this material done by her contemporaries, check out Susannah McCorkle, Sheila Jordan. Alec Wilder, in his landmark book, "American Popular Song," claimed that more than 300,000 songs emerged out of the inspirational triad of Tin Pan Alley, Broadway and Hollywood musicals. Many of these songs, particularly the slower, introspective ballads that formed the core of the repertory, would play a vital part in the continuing emergence of a uniquely American culture. That they are available is a comfort; that they be heard is a necessity.