Of the various theaters on Broadway, the Ambassador is not one whose design cries out for landmark status. But if Barbara Cook were made a permanent part of its interior, I would argue passionately for immediate designation.
For the last 30 years Cook's has been one of the voices that define Broadway. In the 50s, when her career began, musicals were still communal celebrations of American innocence, which her radiant, shimmering voice embodied triumphantly.
In the intervening years musicals lost their innocence and their musical assurance. An instrument like hers was as unnecessary as a Steinway grand in a bawdy house.
Rather than wait for styles to change, Cook has provided her own vehicle, "A Concert For the Theater," a scrumptious evening of song.
Only a few pieces are from shows she has done. There are many treasures by theater composers - Rodgers and Hart, Noel Coward, the Gershwins - as well as pop songs, classics and contemporaries.
Her voice is as resplendent as ever, having lost neither its shimmer nor its agility, which she proves in the eerily sweet sounds she achieves in the higher registers and in the dazzling way she "scats" "Sweet Georgia Brown."
Given the sheer size of the voice it does seem odd that the show is amplified, though the sound design is clean enough. In some ways her most affecting song is "Why Did I Choose You?" with unamplified voice and Wally Harper's eloquent piano.
Between the elegantly arranged songs Cook tells anecdotes in a breezy way, more suitable to a nightclub than the theater. But it doesn't really matter. You come to hear her sing, and that she does, splendidly. Declaring her a landmark would help the uninitiated understand what Broadway used to be about.
If you are among those benighted souls who believe that everything in the musical theater nowadays originates in Britain, you might find unexpected reinforcement at the Ambassador Theater.
Barbara Cook, whose "A Concert for the Theater," opened there last night, is as American as her star-spangled manner, but the show - with her long-time musical director, Wally Harper - originated in London last season.
London, happily, has a special regard for this kind of American songfest - as can be witnessed by the Jerome Kern anthology of last year and the "Side By Side By Sondheim" of some seasons back.
Although Cook's roots are solidly embedded in the moderately terra firma of Broadway's musical theater, she has of late been more of a concert and cabaret singer.
It is surely a pertinent commentary on the Broadway she once graced so steadily that it has not been able to come up with a new musical for her since that ill-fated "The Grass Harp" in 1971. Sixteen years is a long time between drinks and even now, although Cook is back on Broadway, it is still only in concert.
But let's be grateful for small mercies. Now, looking Juno-esque and dressed like an operatic diva - she has three switches of toga during the evening! - Cook tries to change the atmosphere of the theater into a cozy cabaret.
Her down-to-earth informality is, however, too contrived for spontaneity, despite her breezy backstage chatter about stars such as Gary ("Coop") Cooper.
Furthermore, unlike Lena Horne, Peggy Lee or, in a very different way, Liza Minnelli, she never manages to endow her show with any kind of theme or focus.
She clearly hopes to appeal to the kind of audience that once doted on Judy Garland or perhaps Marlene Dietrich, but seems to lack pathos for that particular poignancy.
As a singer she is a wonderful technician - a little like Mel Torme in her complete command of syncopation - with an often thrillingly unexpected accentuation (she tells us she learned how to stress consonants from the great Mabel Mercer) and a Montserrat Caballe-like way of poising perfectly formed notes on a string of sound like an aural necklace.
As a concert artist she also has that Victoria de los Angeles manner - of making everyone in the audience feel that she is singing for him or her alone.
The trouble with her is that just as I find myself defining her through her peers, I find that her own persona - the real Cook in this broth of impressions - is disconcertingly elusive.
The art of this kind of theater concert singing is basically the same as the art of singing German lieder. Each and every number has to be made into a dramatic unit - there is a story to be told, a mood to be set.
This is where Cook fails. A lovely singer, she is simply not expressive enough nor dramatic enough to carry a complete program.
Once in a while, when she returns to her former Broadway hits - nothing, oddly enough, from "The Music Man," not even "Goodnight, My Someone" - she does indeed re-create character. Thus, she was at her very best in "Dear Friend" and "Ice Cream" from "She Loves Me."
She also beautifully recalled her Broadway debut in "Carousel," where she appeared in Jean Darling's original role of Carrie Pipperidge, by singing "Mr. Snow."
Away from her own theatrical past, she appeared less assured. For example, her rendering of that strange, ironic Janis Ian balled "Stars" lacked the musical color and credibility of sentiment that fills Torme's version.
Yet I have rarely, if ever, heard Stephen Sondheim's "Losing My Mind" better performed, and in some of the upbeat numbers, such as "Sweet Georgia Brown," "Carolina in the Morning" and "Them There Eyes" - where she is accompanied by a handsomely bumbling tuba and even ventures a kazoo solo on her own - she possesses an irresistibly unzipped zippiness.
Perhaps what Cook needs most of all is a producer - or, better yet, a show.
It's not just Barbara Cook, now appearing in concert at the Ambassador, who has traveled a long way from the 1950's Broadway of ''Candide'' and ''The Music Man.'' As the passing decades have matured this one-time ingenue, so they've diminished the musical theater in which she made her name. But for everything that has changed, Miss Cook's creamy soprano remains remarkably intact: it's still a lovely instrument whose uncomplicated personality seems as sunny as the happy days that musical comedies used to celebrate. One need only hear Miss Cook's eternally youthful voice connect with the eternal old songs to feel the tingling sensation of years and years - hers, ours, Broadway's - dropping away.
Though ''Barbara Cook: A Concert for the Theater'' doesn't always feature those songs - and sometimes forsakes song altogether for tedious chat - the evening's high points will carry fans of the star and her theatrical tradition past the lulls. Miss Cook's unflagging musicianship honors composers as various as Gershwin, Berlin and Sondheim, and her affection for lyricists, from the Ira Gershwin of ''A Foggy Day'' to the Sheldon Harnick of ''Dear Friend,'' is nearly as intense. Following the elegant example of Mabel Mercer, whom she calls ''the most profound influence'' on her work, Miss Cook knows just how delicately to snare the sardonic payoff in Lorenz Hart's lyric for ''He Was Too Good to Me.'' Another, far more familiar song by Hart and Richard Rodgers, ''Wait Till You See Him,'' proves a revelation in the relentlessly cresting version that sends the audience into intermission.
Unlike many of her peers, Miss Cook understands Rodgers's later collaborator, Oscar Hammerstein 2d, as keenly as she does Hart. The masterpiece of this recital may be ''Mr. Snow,'' the song delivered by the smitten Carrie in ''Carousel.'' Miss Cook's reading mines more wit than one ever imagined in Hammerstein's account of the olfactory inconvenience of being wooed by a fisherman. The singer then ambushes the audience's emotions by charging abruptly to the heart of Carrie's rapturous fantasy of domestic bliss. ''Then it's off to home we'll go,'' sings Miss Cook, unfurling the lyric with closed eyes, her head and voice both yearning heavenward on the word ''home.'' In ''Mr. Snow'' alone, she fulfills her opening-medley promise (via Berlin's ''Let Me Sing and I'm Happy'') to provide songs that provoke both laughter and tears.
The results are less involving if the star strains for dramatic effect. When Miss Cook prays for a ''good-news day'' in ''The Man I Love,'' the good news could just as easily be a paid vacation as romantic passion. Sondheim's ''Losing My Mind,'' a Harold Arlen-style torch song that Miss Cook first performed in the Lincoln Center concert version of ''Follies,'' is gorgeously sung yet not quite as affecting as Dorothy Collins's shattering original. Nor will the alternately overheated and swinging treatment of Arlen's own ''Come Rain or Come Shine'' make anyone forget Judy Garland.
Paradoxically enough, the all-American Miss Cook, who uses a rousing ''Carolina in the Morning'' as a signature number, is at her most moving in a Noel Coward ballad, ''If Love Were All.'' Hard as she works, however, she cannot lift a couple of contemporary exercises in the bittersweet - Janis Ian's ''Stars'' and Rupert Holmes's ''Widescreen'' - above the precious and sentimental. Equally expendable are the professional but generally innocuous comic songs - composed by the star's pianist and arranger, Wally Harper - that fall into the category of what Las Vegasites call ''special material.'' One can't be blamed for wishing the time had been devoted instead to absent, Cook-originated songs by Meredith Willson (''Goodnight My Someone'') and Leonard Bernstein (we get ''I Can Cook Too'' in lieu of ''Glitter and Be Gay'').
Mr. Harper leads an onstage band that, to its credit, is more Cafe Carlyle intime than Broadway brash. But the Playbill lists no director or writer for this ''concert for the theater,'' and both were clearly needed. A raconteur Miss Cook is not, as becomes painfully apparent in her good-natured, unspontaneous and unfailingly pointless anecdotes about backstage encounters with show-biz royalty. Her worshipful description of Mabel Mercer in particular drags on so long that Mr. Harper doodles idle accompaniment at the keyboard, playing the melody of a Jerry Herman song about another Mabel (Normand) that Miss Cook never does sing.
The show's glitches and stylistic conflicts reflect the inherent dilemmas of an entertainment whose soul is divided between the stage and the cabaret, between the unpretentious Broadway musical theater that the star grew up in and the electronic universe of today. The difference between then and now is never more dramatic than at the moment late in the evening when Miss Cook puts down her microphone to deliver a beautiful song by Michael Leonard and Herbert Martin, ''Why Did I Choose You?,'' from ''The Yearling,'' a Broadway failure of the mid-1960's. Enjoying Miss Cook's clarion voice without metallic amplification, one can picture her again in her final important Broadway musical - ''She Loves Me'' - of the same period. ''I lost my heart so many years ago,'' goes the song's lovestruck lyric, and Barbara Cook's admirers will remember that they did, too.