CLASSICAL MUSIC is only a small corner of the total recording field, accounting for perharps five percent of sales in this country, a bit more in civilized areas.

Within that restricted area, this latest and most useful survey evaluates only two decades of stereophonic sound. Despite these restrictions of content, and the further limitation of its scope to records readily available in the United Kingdom, the compilers of The Penguin Stereo Record Guide found that the available material was too abundant to be discussed adequately in a volume as thick and close-printed (though not quite so large) as a major city's telephone directory.

"Pressure of space forbids that every single obtainable LP is discussed within these pages," they explain in their introduction."We have been consciously selective in trying to include virtually everything that is really important. For instance, in the case of Dvorak's New World Symphony we have discussed about two thirds of the available recordings, but even so about 20 records are involved, chosen because they appear to be competitive by reason of quality of performance, the interest of the artist involved, excellence of recording , and of course price. We have sometimes included records of comparatively indifferent quality if the music is not otherwise available, or if a lower price means that readers might reasonably expect to make some allowances."

What is truly remarkable about these listings, however, is not the 20 New Worlds or the baker's dozen of Scheherazades that have been included, but the fact that there are two competing versions of Schoenberg's Moses und Aron , two of Berg's Wozzeck , and even one of Gustave Charpentier's obscure opera, Louise . These are all elaborate, expensive works to produce, and they appeal to an infinitesimal minority within the already small audience for classical music. Their presence is a reassuring sign that someone in the recording industry is looking beyond next month's sales figures.

But they also raise the question that most concerns the nonaffluent lover of recorded music, the question this volume is designed to handle: Which of the 20 New Worlds or the two Wozzecks should I purchase? This dilemma, as well as the added dimension of stereo, is neatly symbolized on the Record Guide's cover, in a cartoon that shows Little Nipper, the dog whose attention to "His Master's Voice" has made him one of the world's best-known trademarks. Nipper has two heads in the cartoon, with the horizontal lines that are a cartoonist's shorthand to convey the idea of motion; he is turning his head from side to side, and over that sits a question mark.

To that question, Messrs. Greenfield, Layton and March offer a neatly nuanced variety of responses. For each of the innumerable records discussed, they supply an instant evaluation in the form of one to three asterisks, rating the performance and sound from "acceptable" to "out-standing," followed by a paragraph of discussion.

In addition, they award a special rosette, very sparingly, as "a quite arbitrary compliment by a member of the reviewing team to a recorded performance which he finds shows special illumination, a magic, or spiritual class." A collection made up exclusively of these recordings would be quite small, very high in quality and reflective of a high eclectic taste. It would include all the Haydn symphonies recorded by Dorati, Chopin's Piano Concerto No. 1 as interpreted by Pollini, a collection of short Delius works interpreted symphonies: the Fifth by Carlos Kleiber and the Pastoral by Otto Kelmperer. And even at this exalted level, sometimes the careful collector might find room for doubts and second-guessing. Is Klemperer that much better than the low-priced Reiner? Bruno Walter's Pastoral is unmentioned; can it be unavailable in England? Does Delius, no matter how well performed, belong in this company? Does anyone who is not a specialist really need more than 100 Haydn symphonies?

There are no final answers, except those that the individual works out for himself; the compliers of this book are working in an area where ultimate decisions are all a matter of personal taste, and it is greatly to their credit that they recognize and accept this fact so flexibly. For the collector who has experienced the music or knows the composer or performer and who has a fair idea of what kind of collection he wishes to assemble, they have prepared a remarkably useful tool. Examining their evaluations, one must concede that this is indeed "the most comprehensive coverage of the recorded repertoire in stereo ever attempted in the pages of a single volume," and furthermore that the judgements it makes available are generally (allowing for variations of personal taste) solid and reliable. A collection based on its three-star listings would be a very fine collection indeed - though here, too, one must choose; otherwise, one would have thousands more records than the average home can comfortably enclose, including four versions of Rodrigo's Concierto de Aranjuez .

There are shortcomings, inherent in the fact that this is a book about a volatile, fast-moving industry and that it had to be complied in a particular time and place. Small American companies that lack over-seas distribution facilities are (understandably but regrettably) neglected. And even on the international scene, events have already begun to outdistance the compilation of them - the single recording of Charpentier's Louise mentioned above, for example, has already been joined by another, equally estimable but too late for inclusion. Still, this compilation is the best of its kind and certainly destined to be a basic reference. Originally scheduled for publication this week, the book has suffered shipping delays but will be generally available by the end of the month.

For the consientious collector wishing to keep up with the field, there are numerous periodicals: High Fidelity, Stereo Review, The American Record Guide and England's very authoritative monthly, The Gramophone. A book particularly useful for American collectors is Records in Review, compiled annually by the editors of High Fidelity and including all of the previous year's classical reviews from that magazine. The latest edition (22nd in the series) is priced at $14.95 and can be ordered by mail from the Wyeth Press, Great Barrington, MA 01230.

Also useful for American collectors who wish to decipher the catalogue listings in The Penguin Stereo Record Guide is the monthly Schwann Record and Tape Guide , a catalogue of records generally available in American stores. Collectors who seek guidance on repertoire will find it in Schwann's 16-page Basic Record Library booklet, and those who wish to concentrate on performing artists as a basis for their collection will find useful information in the Artist Issue that Schwann publishes every few years. These items can be ordered from the publisher at 137 Newbury St., Boston, MA 02116.

Beyond all such aids, however, the most useful tool for the collector is his own familiarity with the music. Frequent attendance at live performances (which abound in the Washington area, including many that have no admission charge) is a good way to learn what you may want to acquire permanently, and a good FM radio is also helpful. Washington's good-music programming is at least equal to that of any other metropolitan area in this country, and it offers the record-collector an easy, inexpensive way to broaden his knowledge.