First of all I want to say that nothing exists about singing that is separate from the people who are out there doing it.
This leads to the logical conclusion that most books about singing technique are irrelevant because nothing in their contents leads back to specific examples of specific singers. If there is no illustration, especially now in the modern world where illustrations are everywhere, there is no content.
I know how to pronounce French because that is how actual French singers pronounce it. This is only one of hundreds of possible examples.
If I can't see it and hear it, it doesn't exist. I don't want to read about how some example in the laboratory or in your studio does something unless you let me hear it, too.