Franz Schubert, ""who is all love,"" has found a faithful lover in Dietrich FischerDieskau: baritone of baritones in his performances of the master's lieder and enthuasiast of enthusiasts in this song-by-song appreciation. ""Biographical?"" Well, yes, the format is heavy with chronological dating; diverse letters and diaries are quoted; and nuggets of mood and incident do launch most chapters. And connections are made--between Schubert's sorrows (doomed romances, family quarrels, venereal disease) and his world-weary, death-wishing musical sentiments. But it's the songs themselves that breathe here, with Fischer-Dieskau stinting on neither scope (600 titles in the index) nor detail. The poets whose work became great lieder receive almost as much attention as the composer--unfriendly Goethe, ""truly mediocre"" Schiller, manic-depressive Mayrhofer (Schubert's roommate), Ossian, Schlegel--and Fischer-Dieskau maneuvers easily in the period's philosophical crosscurrents. He is most sure-footed, however, when immersed in a lied, telling its story, recollecting its piano part, analyzing its intertwining of technique and emotion: ""That semiquaver trill expresses a profound psychological disturbance."" The settings of identical texts by other composers are compared (thumbs down for Hugo Wolf), and the Fischer-Dieskau performing savvy comes repeatedly, but modestly, to the aid of future interpreters. A great singer who is not quite a great writer has overcome his limitations--and those of his pedantic blueprint--with intelligence, humanism, and passion.