We owe some of the most striking and permanently valuable music of the 20th century to a not particularly musical young woman named Kamila Stosslova, who was the wife of an antique dealer in Czechoslovakia about 65 years ago. Kamila entered music history during the summer of 1917, when she met the eminent composer Leos Janacek. It was the beginning of an intense and (for Janacek, unwillingly) platonic love that lasted 11 years, until his death.

Although we are told that she was "a remarkably generous-hearted lady," Kamila gave little encouragement to the great man's advances beyond offering him friendship. Nothing further was meant to be. Not only was he happily married, but she lived in the Bohemian town of Pisek, while he lived several hundred kilometers away in the Moravian town of Brno. Most important of all, perhaps, she was 25 years old and he was 63.

During the years following that first encounter, he became a friend of Kamila's family, helped her husband when he had a problem with the government, and finally died of pneumonia contracted while he was looking for her 11-year-old son, lost in the woods. He also wrote her hundreds of letters, almost one a day for long periods, but above all she was clearly the inspiration for an absolutely dazzling outburst of composition: four unique and magnificent operas, two strikingly original string quartets, a violin sonata, the sinfonietta, concertino and capriccio, the Glagolitic Mass and a suite for wind instruments that he titled "Youth."

Sublimated eroticism was not the only element in this amazing productivity. Janacek already had a strong and strikingly individual musical personality, and an intensely nationalistic spirit (expressed particularly in the Glagolitic Mass) which was given new inspiration by the liberation of Czechoslovakia from the Austro-Hungarian Empire.

Janacek was not exactly unknown before that encounter and its creative aftermath. He had already composed some fine choral and chamber music, his early-1900s opera "Jenufa" was beginning to attract attention, and he had already composed the striking orchestral rhapsody, "Taras Bulba," though it would be revised before its first performance in 1921.

The impress of Kamila is felt clearly in the post-1917 music--most obviously in the vocal music, beginning with a magnificent song cycle, "The Diary of One Who Disappeared," and ending with the bleak, forlorn opera based on Dostoevski, "From the House of the Dead."

Between these extremes, she is seen in several guises: first, in "Kata Kabanova," as the heroine of a tragic love story, a woman married to the wrong man. Then, in "The Cunning Little Vixen," an opera based on a popular comic strip, she is a fox in a charming fable that mingles human and animal characters--a force of nature, amoral and eternally self-renewing. In "The Makropoulos Case," a chilling bit of science fiction (with a text by Karel Capek, who invented the word "robot"), the theme of age and youth takes a macabre twist; the heroine (daughter of a 17th-century alchemist who invented a life-preserving elixir) is now living in the 20th century, condemned to an unending life whose emptiness haunts her.

Janacek's instrumental works found an audience outside of Czechoslovakia long before his operas--not only because operas are much more expensive to perform (and therefore more risky) but because Janacek (like his countrymen Dvorak, Smetana and Martinu) used librettos in a language that few international operatic stars would bother to master.

The key was, of course, the production of accurate, sing-able translations, and that happened slowly. Janacek was almost totally unknown as an operatic composer in the English-speaking countries until after World War II, but in the past few years his works (particularly "Kata Kabanova" and "The Makropoulos Case") have begun to become established in the basic repertoire of major American companies.

Until now, we have also had to depend on translations for adequate book-length studies of Janacek's life and work, but Ian Horsbrugh's thorough and well-written survey fills that gap very well. For specialists, it is somewhat less complete than Jaroslav Vogel's "Leos Janacek," which was published by Norton last year in a revised edition. Vogel had known Janacek and was a highly respected conductor of his music, and Horsbrugh, in his bibliography, calls Vogel's work "superbly comprehensive"--as indeed it is.

Horsbrugh's touch is somewhat lighter and specifically directed to non-Czech readers; he supplies a useful map and other background information to help orient the nonspecialist. He has a sure sense of what to leave in and what to omit in a book that does not aspire to be definitive. And above all, the arrival of his book marks the long-overdue recognition of Janacek as a major composer in English-speaking countries.