This should be a motto of working life for singers.
I do recommend you to do an audition through NYIOP! www.nyiop.com
Auditions are perhaps the most stressful moments opera singers can face.
Both Enrico Caruso and Luisa Tetrazzini are quite objective and we can learn a lot in a few pages.
There are forty interviews to famous opera singers and all about the technique of singing.
Definitive Guide for Opera Singers Auditioning and Working in Germany, Austria, and Switzerland is a wonderful book.