The Adagietto is arguably Mahler’s best-known composition, particularly for American audiences. Although it is often associated with death (Leonard Bernstein conducted it at a memorial for President Kennedy), according to conductor Willem Mengelberg, Mahler actually composed it as a musical love letter to Alma during their brief courtship. Mengelberg wrote, “This Adagietto was Mahler’s declaration of love to Alma! Instead of a letter he sent her this in manuscript: no accompanying words. She und...