The musical theatre of Stephen Sondheim probes deeply into the most disturbing issues of contemporary life. By challenging his audience with intricate music, biting wit, and profound themes, he flouts the traditional wisdom of the musical theatre. Tracing Sondheim's career from his initial success as lyricist for ''West Side Story'' and ''Gypsy'' to his most recent work - ''Into the Woods'' and ''Assassins'' - Joanne Gordon emphasizes not only the disturbing content of Sondheim's work, but his innovative use of form. In shows such as ''A Little Night Music'', ''Sweeney Todd'', and ''Sunday in the Park with George'', Sondheim's music and lyrics are inextricably woven into the fabric of the entire work.