Rupert Holmes, an obscure pop star today, may be the talk of the New York theater world tomorrow.

Holmes -- who has written everything from chart-topping songs to Japanese zipper commercials -- has found a new job.

Working with 18 musicians in a small room within the Public Theater, he is readying his first musical, "The Mystery of Edwin Drood," very loosely based on Charles Dickens' unfinished novel. Not content to write the music, lyrics and libretto Holmes also orchestrated the work, and anxiously watches the conductor direct the players, wishing he could do that, too.

To those who have pigeonholed him, Rupert Holmes remains the Pin a Colada Man, crooner of a frothy story-song that scaled the heights of the pop charts in 1979. Some remember "Him," one of the few personal pronouns to conquer Top 40, and perhaps a few recall "Widescreen," a critically acclaimed 1974 album.

But, for the most part, the quiet intelligence of Holmes' music has long been drowned out by a clan of pelvic synth-wizards. Now, working with "Chorus Line" producer Joseph Papp, Holmes has a shot at Major Composer status, a tag that has thus far eluded him in an overlooked but noteworthy career.

Only a block away from Holmes' new creative domain, the scene at Tower Records in Greenwich Village is telling. A hip lad who calls the store's collection the biggest in the world is asked if there are any Rupert Holmes albums. He looks in the rock section, between The Hollies and The Honeydrippers, and finds Holmes' rack.

The bin is empty.

"Edwin Drood," now in previews, will open Aug. 21 in Central Park. If it succeeds, it will head for Broadway and "legitimize" Holmes. If it fails, Holmes will have spent two years of his creative life on a miss, but one that freed him from the inanities of the pop music business.

"This is the best thing I've ever done," says Holmes. "I had forgotten how lovely it can be to be an artist. I had lost sight. In having to worry constantly about what a program director at a 100,000-kilowatt station will think of a record, and having to figure out how to add enough digital delay, I had forgotten what it's like to sit down and write what will, before anything else, divert you."

Dickens died of a stroke while writing "Edwin Drood," leaving the title character mysteriously absent and offering readers clues that led nowhere. Baffled critics have pored over the manuscript for years to no avail. Holmes has suggested that perhaps Dickens had no ending and his frustration with the book contributed to his death.

When Holmes read "Edwin Drood" as a youth, he imagined Dickens keeling over at his desk when the book ended in mid-sentence. As early as 1970, he began contemplating a musical.

"I love mysteries not because they're mysteries, but because they get resolved . . . The book introduces you to chaos but you do have the hope of resolving that chaos in the end, which is more than you can say about our own lives."

The project gelled in 1983, when Gail Merrifield, the director of the plays and musicals department of the Public Theater, saw Holmes perform at a nightclub. She sent him a note after the show, telling him to get in touch if he wanted to write for the theater. Holmes called the next day and subsequently presented 10 ideas to Merrifield and Papp. They all agreed that "Edwin Drood" was the strongest, and Holmes decided to drop out of the public eye and write it.

Stationed at a drafting table in his Tenafly, N.J., home, he began what would become a 32-song score and 300-page script, very little of which has been changed. "The illusion is that when you walk into that theater it's 1870 and you've been there every week for the last year because this is where you go on a Saturday night. This is your hang-out, and you have a very vulgar good time."

The show features many twists, several of which will be decided by the audience. Holmes has written various endings that may change from night to night, depending on what the crowd chooses by hand vote. Holmes had to write multiple verses for each song and actors must be able to think fast and always be ready for that big solo.

"I've never seen someone so committed," says producer Papp. Since the first reading in February, Holmes has been working long days, rushing from the actors to the musicians back to the drafting table. "He's the first one-man musical-maker," adds Papp, who has pledged to produce "everything" Holmes writes.

In a business where superlatives are mundane, Holmes' coworkers are using even loftier praise than usual. Merrifield flatly says that Holmes' lyrics are the best she's ever heard. "I don't say things like this normally," she adds.

The work has taken its toll on Holmes. His doctor has advised him to sit in dim light for a while after straining his eyes from composing and orchestrating the 650 pages of music. "My hair is considerably grayer and there are certain areas of my scalp where you could go skating," he says.

He is an "Episcopalian Woody Allen," half British altar boy, half American Jew. His voice, reassuringly placid in tone, only reveals a glint of his mother's British birth -- when he says "rah-ther" in that smooth radio announcer's voice. But when he's about to crack a joke, a slight stammer with that cynical Manhattan edge comes through.

A prodigy on the classical clarinet, fathered by a classical musician who also played jazz and mothered by a devotee of Noel Coward and Rodgers and Hart, Holmes grew up in Nyack, N.Y., loving the Beatles and Bob Dylan but respecting the masters -- Ira Gershwin, Arthur Schwartz and Howard Dietz, and Cole Porter. He recites their lyrics lovingly.

He won a four-year scholarship to the Syracuse University School of Music, but left after a year, longing for New York. Two years at the Manhattan School of Music followed, but he eventually quit as he gravitated toward pop. "Classical music always made me feel like I was wearing a tweed suit without underwear. It was very, very itchy."

Ambitious and talented -- he plays at least 17 instruments -- he got his big break as a $75 office boy/exclusive songwriter for one of the music publishing firms in the famed Platt Building on Broadway. Holmes has joked that he could have made $100 a week as an office boy if he hadn't insisted on also being hired as a songwriter.

Only 10 short years later, Holmes had the No. 1 song in the land. "I hate being Mr. Pin a Colada," he says. "It makes me sound like a guy who dispenses frozen yogurt. First of all, musically, it's probably the blandest thing I ever wrote. Secondly, unfortunately, a lot of people don't listen carefully and they think it's a song about how nice pin a coladas are."

"Pin a Colada" may be repetitive and melodically simplistic, but the song typifies Holmes' talent for storytelling: A man bored with his lover scans the personals and is intrigued by a woman who likes pin a coladas and getting caught in the rain. He answers back with his own witty poem and the two arrange a meeting in an Irish bar. Of course, the woman turns out to be his current lover.

The song is also a good example of how seriously Holmes takes his craft. He had written four previous sets of lyrics, and when he finally arrived at the last version, it didn't include a reference to pin a coladas. Instead, the refrain went: "If you like Humphrey Bogart, and getting caught in the rain." Tired of movie allusions, Holmes set his sights on a tropical beverage. "Some stupid, overripe drink. And I thought, 'Let's think: stinger, scorpion, mai tai, pin a colada. Pin a colada. Right number of syllables.' "

The three LPs that followed his "Pin a Colada" success didn't sell nearly so well. Insiders who worked with Holmes say he lacked the stage presence to create excitement and attract a big audience. With a laid-back, unadorned singing voice and songs that required more than a few samplings to appreciate, Holmes didn't catch on.

In its latest anthology, Rolling Stone dismisses Holmes' career, which has spanned seven albums and about 15 years, in fewer than 60 words, describing him as someone aspiring to be "Barry Manilow with class."

If T.S. Garp had been a songwriter, he would have penned lyrics like Holmes', which are full of witty, white-collar fantasies: "He is a buyer for Bloomingdale's. She's division head of commercial sales," he sings, adding, "Work is the great aphrodisiac. It's that 9 to 5, gets 'em in the sack."

Among his other lyrical epics: "Terminal," in which two Wall Street commuters meet on a bus and head to a friend's house for an afternoon tryst. The protagonist's brief moment of bliss is shattered when he realizes his responsibilities and sings: "Could have held her body my entire life, but I had to get home to the kids and the wife." "Our National Pastime," in which the sleazy Warren Hitler tries to pick up a blond at Shea Stadium. The song includes a minute-long dialogue in the man's bedroom, baseball special effects, and a stirring arrangement of the National Anthem at the height of their sexual encounter.

He writes about falling in love with people in passing Buicks and planes you've just missed and bus stops where your bus will never stop:

Or you'll share an elevator, just you two.

And rise in total silence to your floor.

Like the fool you are you get off

And you leave your life behind a closing door.

"I fall in love at the drop of a hat and I bring a hat with me so I can drop it," he says, adding, "But that doesn't mean I do anything about it."

In his first album, "Widescreen," he wished life could be as exciting as the movies, that women would say the right words on cue and lovemaking would be accompanied by a lush orchestral score. "Life's a constant disappointment, when you live on celluloid," he sang. "But my movie expectations are the dream I can't avoid."

Ironically, Holmes, a man who was shy about his appearance -- who used to get nauseous before concerts -- soon found himself living the glamorous life he imagined. Barbra Streisand heard "Widescreen," and within a year he was living in her California guest house, coproducing her album and developing the music for "A Star Is Born." He even had his own office at Warner Bros., and would stroll by the very sets he'd sung about.

But he soon grew tired of the California scene, and he now believes reality is just as exciting sans Technicolor and Dolby stereo. "The great adventures are not necessarily Bruce Lee or Clint Eastwood. The great adventures are that we make it through life, or that you call someone, get a wrong number, and continue the conversation because they sound interesting. That is adventure, just the silly little moments of our lives."

"I've been to Hollywood, I've seen the sets, and it's not any better than what we're doing in a coffee shop right now," he says.

It's hard to believe that a man who has devoted entire albums to countless love affairs has been with his wife and 9-year-old daughter in suburbia for years, but it's true. "He dreams," says friend and manager Normand Kurtz. "But he doesn't just dream. He dreams fantastic dreams."

He occasionally finds himself ignoring the headlines and retreating to his music, books and movies. His idea of "total debauchery" is walking into an air-conditioned movie theater on a sweltering day. "That's a wonderful thing, because the world is suffering and you're in this other world."

Holmes often is in another world. He loves Victorian England, has researched it thoroughly, and is the first to admit he prefers his version of England to the truth. He knows the famous London fog was nothing more than pollution from the Industrial Revolution, but he still loves to escape into that mysterious mist.

That yearning for the past isn't limited only to "Edwin Drood." He collects old films and tapes of the radio soap operas of the 1930s -- he even put a 10-minute mystery show on his first album. He also delights in announcing that "Brass Knuckles," a rhyming, Chandler-esque whodunit, was the first pop song reviewed in Ellery Queen Magazine.

He can recite entire movies from memory and has written a bunch of songs about classic horror characters. "He can look at a black-and-white movie for 30 seconds and tell you the entire cast," says his wife Liza.

The past has influenced his musical style as well. He belongs to the old school of songwriters, who rhyme and arrange their songs meticulously. He'll be contemporary, but he can't let go of the American song-writing tradition. He describes himself as someone who would have led the French Revolution, but in the midst of the uprising, as the buildings were burning, "I'd be the idiot running up and down the hall trying to save the paintings."

Few things about song writing satisfy him more than capturing people's feelings and experiences. On two of his albums, he asked his listeners to write to him, and wound up personally answering hundreds of letters. "Most people I think go through life thinking they're nuts and they're crazy and they're alone in what they feel, so they cover up . . .Well, we're all bluffing."

Right now, Holmes says he is bluffing his way through "Edwin Drood." But he is surrounded by a talented troupe of veterans, including Betty Buckley from "Cats," Cleo Lane, the foremost interpretive singer in Britain, and a slew of artists from Papp's last Central Park bonanza, "The Pirates of Penzance": actor George Rose, director Wilford Leach and choreographer Graciela Daniele. Kurtz bluntly states the play will head for Broadway after its stint in the park, but that hinges on the reception it receives.

Holmes knows what he wants from "Edwin Drood," and it isn't money, fame or reverence.

"My loftiest goal in this thing is that if it could make you forget you have root canal work tomorrow. That would be a great achievement."