While it's the tunes we remember, what the musical play must have as a firm foundation is the book.

Just opened in New York is the new Jerry Herman musical, "The Grand Tour," and it -- along with three more million-dollar-plus creations tuning up to scores by Burton Lane, Stephen Sondheim and Richard Rodgers -- could halt the slide of the American musical theater into costly oblivion.

Readying for Wilmington and the Kennedy Center before Broadway is "Carmelina," with a book by two masters of the craft -- Joseph Stein, who adapted Sholom Aleichem's Tevya stories into "Fiddler on the Roof," and Alan Jay Lerner, who did the unlikely by turning Shaw's "Pygmalion" into the inimitable "My Fair Lady." It's a heady but also daring duo.

Hugh Wheeler, who has crafted such highly novel books as "A Little Night Music" and "Pacific Overtures" for director Harold Prince, goes back to a 19th-century crime for "Sweeney Todd, the Demon Barber of Fleet Street" with Stephen Sondheim again his partner for music and lyrics, opening March 1 in New York.

Thomas Meigham and Martin Charnin, who did the book and lyrics for "Annie," are embarked on "I Remember Mama," to a Richard Rodgers score. Their foundation is the play and resulting film and TV series that developed from John Van Druten's "I Remember Mama," which he, in turn, created from a collection of short stories by Kathryn Forbes titled "Mama's Bank Account." This tunes up in Philadelphia before an April Broadway bow for Liv Ullmann in her first musical.

That our musical plays are adapted from previously effective sources is hardly surprising. Most are. During Monday's sensationally moving "Live From Lincoln Center" recital by Joan Sutherland and Luciano Pavarotti it was fascinating to be reminded how many long-lived operas of the 19th century stemmed from popular, melodramatic contemporary plays. The source of Herman's "The Grand Tour" also inspired a modern opera with music and libretto by Giselher Klebe for the Hamburg State Opera. There was little excitement when the German company presented it in New York 12 years ago.

This source was a play by Franz Werfel, which S. N. Behrman reworked for wartime American audiences under Werfel's title, "Jacobowsky and the Colonel." The new adaptation is by the experienced Michael Stewart and newcomer Mark Bramble.

It's the story of a sharp little Jewish wanderer in Europe after the Nazi conquest of France. Jacobowsky's inborn survival kit helps a snobbish Polish officer convey vital information to Free Poland forces in Britain. With this unlikely odd couple as protagonists, "The Grand Tour" is an escape yarn rooted in character, a vital aspect of musicals' books.

Joel Grey has his best role since "Cabaret" and does his finest work since as Jacobowsky. With a trim beard and those dark, daring, sensitive eyes, he swiftly establishes himself and the character with one of Herman's most melodic inspirations, "I'll Be here Tomorrow."

As this little conniver's bumbling opposite, the admired Ron Holgate, large of frame, big of voice, finally, gets a part which fits him. He's a perfect foil, and in time they come to an enchanting, amusing song-and dance duet, "You I Like."

The start is immensely effective, but for Act I's final half hour Stewart and Bramble have imagined a plot development intended, I assume, to widen the focus of Werfel's story. The writers involve their odd couple with a traveling carnival show. The development is awkward, the staging no better. This invention seems painfully old hat and deals Act I a blow from which not even Herman's "One Extraordinary Thing" can save it.

"The Grand Tour's" Act II is as solid a second act as any musical boasts, a remarkable recovery. This time interpolation of a large-scale scene, a Jewish wedding hidden from the occupying Nazis, fits the story and inspires Donald Saddler for several choreographic turns at which star Grey is in Brilliant form. Each of the six scenes and Herman's score proples the story. If only "The Grand Tour" didn't have to lick that lagging first act!

To some of the reviewers those quicksands were fatal, but there have been enthusiastic notices from several major critics.

Given a chance for the word to get around, "The Grand Tour" deserves hit status. Grey's performace is a gem, a novel character employing both the acting skill and song-and-dance personality of its star. "I'll Be Here Tomorrow" is a Herman tune that will last. "You I Like," "I Belong Here," "We're Almost There" and "For Poland" add up to more solid numbers than many favorites boast. Columbia's coming recording should help spread the word.

Like "A Little Night Music," which worked, and "The Baker's Wife," which didn't, "Carmelina" was born as a movie titled "Buena Sera, Mrs. Campbell," which starred Gina Lollabrigida with Phil Silvers, Peter Lawford and Telly Savalas in 1969. It was about an Italian girl who used her World War II illegitimate daughter to convince three American GIs that they should send her support allotments for life. Comes a reunion.

Lerner and Stein can be counted on to give this hardly new situation a fresh twist and one clue may be in the presence as costar of opera basso Cesare Siepi. Especially welcome will be the return of Georgia Brown, who introduced "Whenever He Needs Me" so memorably in Lionel Bart's "Oliver."

For Rodgers, "I Remember Mama" has a long, intimate appeal. Daughter-composer Mary enthused over the Kathryn Forbes stories and passed them along to her mother and Mrs. Oscar Hammerstein II, hoping they'd influience their husbands. Because the collaborators then were busy, they play whcih they then produced. It went on to films and TV, and it's taken the "Annie" writers to create a new shape for Rodgers' score, delayed 35 years since Mary's boost.

In Wheeler's case, an Ingmar Bergman film and a Harvard-Yale student's -- John Weidman's -- study of America's arrival in Asia were the roots of "A Little Night Music" and "Pacific Overtures." While their originality put off general popularity, those of us who admired them were relishing their novelty of concept. So it's reasonable to expect a fresh approach to "Sweeney Todd."

The thousands of musicals which have failed are conspicuous for one thing. Despite good scores and major stars, they had poor, unworkable books.

There are no rules but it's also striking that the best musical books offer a variety of characters which can inspire a composer's versatility and moods. Most of those characters should command audience empathy, the ability to see oneself in a like situation. More than one plot can then result with further calls on the composer's ingenuity.

The lines themselves must be to the point, for time must be found for the musical numbers. A minute can seem like hours in the world of bad book musicals.

The librettists of these four new works don't have to be told these strictures for, at one time or another, all have followed them cagily. No matter how rich the melodists' contributions, this $5-million quartet of entries will fail if their book foundations are faulty. And with their failure can come an end to big, expensive, ambitious musicals.

If the structures for which the music becomes the embellishments are firm, these might reverse the present alarming slide of the American musical.