Percy Grainger's day may yet come. He was not just a brilliant pianist but a progressive, abidingly serious composer. Today he is best remembered, however, for such "fripperies" (his term) as "Country Garden" and "Molly on the Shore," and for his genial folk-song arrangements. Born in Melbourne, Australia, in 1882, he spent much of his extraordinary youth living in London, giving concerts throughout the world, savoring the professional stimulation and personal support of such models as Edvard Grieg and Frederick Delius, enjoying the emphatically requited love of his doting mother and exploring dark, forbidden realms of sadomasochism with a young woman from Denmark named Karen Holten. All of this is chronicled--with graphic detail, lucid prose and often probing insights--in letters he wrote between ages 19 and 32. Kay Dreyfus, curator of the Grainger museum in Melbourne, may focus on the narrative trees and slight the interpretive forest, but she provides helpful annotations and generous period illustrations (including proud portraits of Grainger and Holten in the nude). The bleakness of Grainger's later life--he died in White Plains, N.Y., in 1961, bitter and neglected--no doubt will be reflected in a further publication. For all its revealing documents, this one must appeal primarily to a predisposed readership.