This is the paper that started new school singing.
between-the-looking-glasses.pdf
And here are some more free things to read.
Jack Dempsey.
jack-dempsey-championship-fighting.pdf
Manuel Garcia
Sun-Tzu
Enrico Caruso and Luisa Tetrazzini
https://archive.org/details/artofsinging0000tetr
Nonlinear source-filter coupling in phonation: Theory
Nonlinear_source-filter_coupling_in_phonation_Theo
other reading
Your voice: applied science of vocal art, singing and speaking by Douglas Stanley
An Analysis of the Vocal Teaching Techniques of Alan Lindquest