It took a bit of creative sleuthing to find basic information about Giovanni Clerici (1861-1924), a highly regarded singing teacher in London. 

Conducting a careful search using Google Books and historical newspapers, I found Clerici's death and birth date after entering his name and the city of his last known residence, which happened to be Torquay, England—known as the "English Riviera."

Such an interesting fellow, Giovanni Clerici. He wrote a one act operetta—Lorraine—which was given its premiere at the Theatre Royal in Torquay in 1898, and also one of the more interesting and enjoyable books I have come across.

While I haven't yet been able to ascertain who Clerici studied with yet—assuming, of course, he studied with someone, his book—Perfection in Singing (1906)—contains the kind of advice one expects from an old Italian school maestro. It's also excellent advice too. 

One of Clerici's students was a Jamaican gentleman by the name of Louis Drysdale, who sang in England as a member of the Kingston Union Choral, and rather than return to Jamaica, stayed in England and took lessons with Clerici and Gustave García. "Dri" as he was called, became a very good teacher in his own right—his most famous student being a young Marion Anderson.

Perfection in Singing is very much worth your time—Clerici's opening gambit alone worthy of attention. 

To the Teacher

Cultivate all sorts of desirable moral qualities.

Thus, be gentle, yet firm; genial yet dignified; eager yet patient; learned but not pedantic; versatile but not delusory; systematic but not inflexible; reverent to all classic things, but not bigoted or narrow towards all new things that are good. Finally, cultivate a kindly and friendly relation between yourself and your pupils. Make them feel as though you are taking them by the hand as children, to travel into a land of "fairy," as Spencer said:

"Music does demand much toil and many weary hours of drudgery from her votaries, but how wondrously does she reward the brave and faithful who endure her trial.

While notions of fairyland may make the reader smile, that same reader should not forget how music made his heart open, soar, and even brought him to tears. That it has this power is certain. Learning to wield it is a whole other matter. 

Perfection in Singing is an excellent resource.