VLADIMIR HOROWITZ PLAYS MOZART . In 1996, German author Lea Singer read that a man named Nico Kaufmann had, before his recent death, donated a bundle of letters from his teacher, Vladimir Horowitz, to a Swiss archive. Singer had heard Horowitz perform in Berlin ten years earlier and was struck by a certain sadness in his shy smile. But when she checked a biography of Horowitz, not a word was mentioned about a student in Europe. Many more years passed before Singer got the idea to research these