“The way a composer achieves popularity is as mysterious to me as the way a book becomes a best-seller,” says Thomas Scherman, 36-year-old founder and conductor of the Little Orchestra Society of New York. Music, especially unfamiliar music from the time of Gluck and Purcell to contemporary composers, is Scherman's absorbing interest. But it is natural for him to compare the musical and literary worlds, since in a sense he belongs to both: his father is Harry Scherman, president of the enormously successful Book-of-the-Month Club, the most successful organization of its kind in America.