I am currently cataloging my books, 460 and counting, and have come across a wonderful book by Henry Pleasants called The Great Singers. Mine is the paperback edition from 1966, but I see there is a later edition from 1985 that includes Pavarotti. I don't have the newer edition to compare with mine. It says it starts with Jenny Lind while mine goes back to the dawn of opera. Jenny Lind is a little late in the story.
In 1966 Maria Callas and Joan Sutherland had just begun the bel canto revival, so he is basically writing from the perspective of my youth when opera singers followed the verismo / Wagnerian path. Many of my ideas about the development of technique over time derive from this book. He uses sources. Imagine that. This book is highly recommended.