THERE IS no Gesamtausgabe ," said Mr. Cooper the piano teacher, and at that moment the outline of a life's work was born in the mind of 13-year-old Rob Landon.

In the 39 years since he asked his teacher "What's a Gesamtaugabe? ," the search to create one has led H.C. Robbins Landon into the dank cellars of Dracula-style Hungarian castles, obscure libraries, auctions and antiquarian bookstores - even into monasteries where he has joyfully leafed through the wine-stained-manuscript pages of a 200-year-old string quartet.

"Complete works," answered Cooper. "You know, like the complete works of Shakespeare. They have a Gesamtausgabe for Bach and one for Beethoven - even one for Mendelssohn, but none for Haydn."

It was 1939, in the small Massachusetts town of Lancaster where Landon grew up. The boy was wondering what he would do with his life, and in answer his teacher played him Sir Thomas Beecham's recording of Haydn' Symphony No. 93.

Landon was overwhelmed by the music, and he could not help wondering: If this were No. 93, how many more were there like it? There are 104, he was told. (Today, through his efforts, three more have been discovered.) Could he hear them all? Could he get a recording of, for example, No. 26? Cooper shook his head sadly. If he searched all the record shops of Europe, he might find recordings of about 20 of the Haydn symphonies. Most of the others had been unheard for nearly two centuries; they were unknown, unpublished and many were probably lost.

This music should be available, young Landon decided, and he set out, with the single-mindedness of those who accomplish great things, to make it happen.

"I have been very lucky," said 52-year-old Landon during a recent visit here, looking back on the 13-year-old whose desciplined enthusiasm made him the most spectacularly successful musicologist for our time. "The Haydn phenomenon is something that will never happen again. I was in the right place at the right time to help it happen."

The right place was Vienna and the right time, as it turned out, was 1947. Landon managed to be there, but it was hardly a matter of luck. After World War II, he got to Europe as a music journalist, covering the European musical scene for a radio network, "a weird outfit called the Intercollegiate Broadcasting System."

Austria in 1947 was no place for a civilian, Landon quickly noticed; it was an occupied country, still in a chaotic condition and suffering severe food shortages. "They didn't want tourists and they didn't want musicologists," Landon recalls. "You couldn't live in Vienna in 1947 unless you had an acceptable reason for being there, so I decided to volunteer for the army and they took me in like a shot. I was a private and I wore a uniform, but I enjoyed a sort of civilian status. I had to write a history of Mark Clark's Italian campaign - I suppose it's gathering dust now in some cellar here in Washington.

"I had a nice sort of old colonel; he used to say, 'Landon, I've been a good soldier all my life; what did I ever do to deserve you?' He heard about the Haydn masses we were performing and called me in: 'Landon, what's this I hear about you playing drums in church?' I explained to him about 18th-century masses with full orchestra - including drums - and he told me, 'Well, all right, but don't do it in uniform.'

"I had been admitted to go to Harvard and get my Ph.D.," Landon recalls, "and I thought this over, and I said I'm not going to do it. I'll get myself into the army here so I can stay in Austria and get my hot little hands on those manuscripts. It was going to take me seven more years to get that degree. Harvard was insulted beyond belief; nobody had ever said 'I'm not going to come."

He started out with photographic equipment, bought from army surplus, to track down and copy manuscripts. "The first thing I had to do was get the sources organized; until we found out what's there, what's genuine and what's spurious, we couldn't do anything properly. The second idea I had was, as soon a we got things going sufficiently, we would start a complete edition, which we did, and we would start our own recordings, because I wasn't going to try to talk those idiots (See LANDON, F5, Col. 3> at RCA Victor into it; they'd only say no, nobody will buy it. After two years, in 1949, I came back with our first tapes.

"The only people who were making LP in those days were Columbia, so I went down to Columbia and I said 'Will you process this?' and they said 'Sure,' they were delighted; they thought it was good for business. Then they listened and they said, 'You mean you guys are going to try to sell Haydn as a commercial operation?'

"We issued the 'Nelson Mass,' which was a rave best-seller, we issued the first recording of 'The Creation' and people wept in the record stores - hard-bitten New York sons-of-bitches with tears streaming down their faces; they's never heard anything like it in their lives. And that was the beginning of this marriage between the LP and Haydn which has really changed the face of music history."

He makes it sound a lot easier than it must have been, though no less spectacular. According to legend, a schoolboy once answered an examination question: "Tchaikovsky composed three symphonies: Nos. 4, 5 and 6." Asked about Haydn, in the pre-Landon era, that same schoolboy might have said that he composed 104 symphonies: Nos. 45, 88, 92, 94, 101 and 104. The rest was mostly legend, hearsay - and a curiously condescending legend for one of the most productive geniuses in the history of music or any other art, the man who took the symphony, the string quartet, the piano sonata from an embryonic condition in which nobody quite knew what they were and developed them into the basic forms which would dominate European music for more than a century.

Part of the Haydn problem was the sheer volume of his work: 107 symphonies (one of which was discovered in the Library of Congress), 65 quartets, 62 piano sonatas, 45 piano trios, approximately 40 string trios, 60 divertimentos, 125 works involving the baryton (an obsolete relative of the cello which was played by one of Haydn's noble patrons), 14 masses, 4 oratorios, innumerable cantatas, songs and choral works, about 15 keyboard concertos, 450 arrangements of Scottish songs, and many more works, some of them lost.

"There are about 20 operas," says Landon. "We're still not quite sure how many, and for some of them we know only the title. We're still looking for a lost double bass concerto that every double bass player in the world hopes we will find. Besides the numerous concertos that exist, we know there are lost concertos for flute, for horn, for two horns, for cello and for violin."

Besides the Haydn works that have been lost, the search is complicated by others that should be lost and aren't. Once Haydn's reputation became established, publishers joyfully pirated his works, sometimes "improving" them, and also cashed in on his fame by putting his name on works he did not compose - Leopold Mozart's "Toy" Symphony, for example, Haydn helped future musicologists by compiling various catalogues of his own works. But these lists are not complete, so the evidence is only partial; a work can be genuine but still not be on the list.

To weed out the true from the false, a researcher needs to be familiar with the various musical styles of the period (which was a stylistic crazy-quilt) and with the handwriting not only of Haydn but of those who regularly copied his authentic manuscripts. Frequently, a manuscripts is written in an unfamiliar hand but has corrections in Haydn's handwriting, and these are an assurance that it is an authentic copy.

Where does one go to look for a Haydn manuscript?

Some of the places are obvious, others surprising, according to Landon: "Haydn's works survived principally, besides obvious places like a Library of Congress that collect them, in monasteries and the collections of the nobility of the Austro-Hungarian Empire and Southern Germany - also in other places but principally there. Particularly vocal works like the operas; if they survived at all, they survived in places like that.

"For instance, in 1935 an antiquarian named Arthur Rau offered for sale contemporary copies of two Haydn operas, one called 'Philemon und Baucis' and one called 'Die Feuersbrunst,' that he got from the remains of a big French country house - the volumes were bound with a big coat of arms. 'Philemon und Baucis' went to the Conservatoire, where it remained unlooked-at and unnoticed until 1950, and 'Die Feuersbrunst' went to Yale, where it remained unlooked-at until I ordered a microfilm and realized it was a lost Haydn opera, which he had described in his catalogue. He had another one listed with these two, 'Didone Abbandonata,' which, despite its Italian title, is a lost German marionette opera. One copy of the printed libretto exists. There are other copies of the other two operas; they survived in only one copy in a French country house. Similarly, all over Austria - the Frankenstein-Dracula country - there are these really spectacular, dank, frightening castles that have Haydn manuscripts.

"One of them is called Harburg - that's in Bavaria, and you can see the bats swarming out of it sometimes - and it has a huge collection of Haydn and lots of works by other people. The Prince who owned it was called the Prince von Oettingen-Wallerstein, and Haydn, was in contact with this prince and sent him manuscripts, and the prince also did his own collecting. What the prince couldn't get from Haydn he got through agents in Vienna and the result is an absolutely unique collection of Haydn symphonies there - something like 90 of them.

"The more different copies you have, the better - it establishes a kind of a pedigree - yoy check variations in the text and sometimes you can tell which manuscript was copied from which other. The monasteries used to do a lot of copying - music manuscripts were very expensive, so they would borrow one and have a monk copy it. The monasteries used to play Haydn quartets during meals - you can often see the stain on the back page where a wine glass was set down on top of it - and they would play the symphonies sometimes during mass rather than an Offertory or Gradual. The Organ Concertos were written for this kind of use."

A lot of his research has been done in countries that now have Communist governments - particularly Hungary and Czechoslovakia - and he recalls their cooperation with a special warmth: "Without the Hungarian Communists, it would not have been possible; the Eszterhazys would not have let me go poking around their castle."

Part of the job would also have been impossible without Hungarian non-Communist Antal Dorati, who managed to record the complete symphonies in collaboration with Landon after two previous efforts (Landon's own Haydn Society and the late Max Goberman's Library of Recorded Masterpieces) had fallen short. The complete set came to 48 long-playing records, issues by Decca in England, and London Records in this country. "The Americans were still skeptical about Haydn as a commercial venture," says Landon. "When they finally agreed to cooperate, one of their executives wired to England: 'We will accept your wallpaper music.'"

Since then, the Dorati set has achieved gold record status, with more than 2 million records sold - a very rare distinction for classical music and particularly amazing for such an obscure and bulky set. "As long as the industry was in the hands of those 19th century oriented characters, it was a typical vicious circle," Landon observed. "They were never going to record any Haydn because they knew Haydn wasn't going to sell. When Dorati finished the symphonies, he told me, 'Now we will record the operas.' I told him, 'They won't take them.' Dorati said, 'They'll have to take them; I am the original man in the revolving door.' I said 'What's that?' and Dorati said, 'Don't you know the definition of a Hungarian? A Hungarian is a guy who goes in behind you in a revolving door and comes out ahead of you."

Dorati was right.

So far, Philips has released recordings of four previously unknown Haydn operas. One of them, "Orlando Paladino," has already sold an amazing 40,000 copies, and a Hungarian company is beginning to issue competing versions.

Now, says Landon, with an air of obvious satisfaction, "everybody wants to get on the bandwagon. French Decca came to me and said, 'Everyone else is doing Haydn and we don't have anything,' so I said 'I'll give you the string trios; I'm editing those now,' and that made them happy."

As a sort of by-product of his Haydn research, Landon is producing a massive biographical work: "Haydn: Chronicle and Works," which is being published by the University of Indiana in five volumes and will total nearly 4,000 pages. It was begun with Volume III: "Haydn in London," which covers the period of the composer's life that is of greatest general interest, with copious illustrations, reproductions of all important documents related to the life, a biographical narrative and a critical analysis of the music produced during the period. It is, as it has to be, very expensive ($45 per volume), and publication will not be completed until 1980.

"They were wonderful," according to Landon. "They never argued with me, never said, 'Please take out the color, please let's have fewer illustrations, please cut the text or the musical quotes. They really knocked themselves out to make it right and they took a terrible risk, and you know, they've almost sold out 'Haydn in England.' In six months, there won't be any more copies."

Meanwhile, the Gesamtausgabe , where it all began, has been handed on to others. "The Germans are working on that now, and they're the ones who should do it. The volumes are coming out very slowly, they're very beautifully done and very expensive. It will take until the year 2000 to finish.

"My editions are scholarly, but they're not real critical editions with the full critical apparatus, they're really editions for performers. I didn't think we should have to wait that long for all this music."