This is meant for pianists, but there's some essential stuff in there for singers, too.
Establishing your personal library
The Fischer-Dieskau Book of Lieder and Bernac French/Poulenc songs are a good start.
I'm in a pedagogy graduate course right now, this book discusses hormones, aging voice, medications, etc. if you want to have lots of info: https://www.amazon.com/Singers-Guide-Complete-Health/dp/0195374037
the Feldenkrais technique has been a lifesaver.
A good book for Feldenkrais exercises for singers is called Singing With Your Whole Self - https://www.amazon.com/SINGING-YOUR-WHOLE-Samuel-Nelson/dp/0810840499