FRENCH FOR the act of rendering the esoteric understandable to the public is vulgariser - a word whose amusing overtones in English too often are appropriate to good-natured attempts to bring the lofty business of classical music down to earth. Robert Harris, a CBC radio executive, has more serious aims in What to Listen For in Mozart. How serious? Froth page 50 to 57 he reprints the score of the first movement of the popular serenade known as Eine Kleine Nachtmusik as a study aid to a thematic analysis that begins on page 21 and continues, with digressions, to page 59. This may sound more like the stuff of an undergraduate text than a layman`s primer, but I think Harris has found an apt high-middle course for readers who are willing to concentrate, bear distinctions in mind, and accept complexity for what it is. The choice of composer, of course, was not haphazard: 1991 marks the bicentennial of the death of Mozart and the inauguration of a commercial campaign that promises to displace Beethoven in the public imagination as the music genius par excellence. There are many substantial tributes planned, and television specials abound. Critics and hardcore music buffs have viewed all this with suspicion; death anniversaries are not normally observed so lavishly. Yet Mozart`s music has undoubtedly grown in popularity among non-expert listeners since the release of the 1985 film Amadeus. Harris has made it his job to explain why Mozart`s art is as much a struggle against the 18th-century style galant as a reflection of it. "Perhaps we of all generations," Harris writes, "are most able to appreciate Mozart`s art for what it is" The approach he adopts is accordingly balanced and comprehensive. His analysis of Eine Kleine Nachtmusik functions splendidly both as an unfurling of a specific masterpiece and an introduction to the principles governing great Western concert music. `A set of circumstances has to be established, challenged and re-established," Harris writes before embarking on a discussion of sonata form. Such global observations are seldom considered by classical buffs, who take rudiments for granted. Harris`s score studies (he deals in comparable depth with the Haffner Serenade, the first movement of the Piano Concerto No. 21, and the finale of the "Jupiter" symphony) refreshed my interest in much music with which I was pleased to think myself thoroughly familiar. My eyes danced more rapidly over pages early in the book outlining the roles of melody, harmony, and rhythm, although these also seemed admirably thorough. Yet through all this groundwork Mozart remains the focus of the book - not such an easy accomplishment, given that the character of the composer has seemed to many people small and vague compared to the music. Harris admits that certain aspects of Mozart`s personality and art cannot be reconciled, but his portrait is more vivid than most. The composer`s conflicts with his father Leopold, a musician firmly aligned with the age of absolutism, are well documented and sagely assessed. The reader is also left with a strong sense of how Mozart was failed by the societies of Salzburg and, later, Vienna. Harris sees Mozart`s declining fortunes in the late 1780s as a natural consequence of his increasing independence as a musical genius. "Freelance composer," alas, was a job description for the 19th century. This is by no means the only biographical point Harris makes. Indeed, the images (Mozart the idiot savant, Mozart the product of his time, Mozart the rebel, Mozart the playful child) sometimes swirl confusingly. The book`s prose style is a little soft and prone to redundancy; we are advised so often of the "charm" of Mozart`s music that the attribute becomes tiresome. There are some debatable details; certainly the author`s decision to refer to the Piano Concerto No. 21 as the "Elvira Madigan Concerto" is misguided populism. (Has anybody actually seen that movie in the last 15 years?) But for its thoroughness and open-minded passion for its subject, Harris`s book deserves success.