 He was also responsible for introducing Sherrill Milnes, Ruggero Raimondi, and Montserrat Caballe.He is a highly regarded expert in the field of opera with an extensive knowledge of the history of vocal music, and his books include volumes 1 and 2 of "The Record of Singing" and "The Great Caruso".In 1992, he also wrote "Maria Meneghini Callas", being one of the few Maria Callas biographers to have seen her frequently on stage in her prime.