There's a fun little book called Caruso and Tetrazzini on the Art of Singing, and I have no idea whether it's still in print. But it's fun to see how ideas about singing have changed (or not?) over the years -
Here's a little nugget from Signor Caruso's section:
"Others, who have keen and alert minds and voices of fine quality, yet lack that certain esprit and broadness of musical outlook required in a great artist. This lack is often so apparent in the person's manner or bearing that I am tempted to tell him it is no use before he utters a note. Yet it would not do to refuse a hearing to all these misfits, for there is always the chance of encountering the unknown genius, however rare a bird he may be".
Interesting take, don't you think?