Robin Hull reviews some recent books on music, including: ' Sibelius-a Symposium,' edited by Gerald Abraham ; ' On Music and Musicians,' the letters of Robert Schumann ; ' Ysaye, his Life. Work, and Influence,' by Antoine Ysaye ; 'Albert Roussel ,' by Norman Demuth ; 'A Companion to Mozart's Piano Concertos,' by Arthur Hutchings ; and ' Bach,' by Eva Mary and Sydney Grew, in the Master Musicians series