One day this decade I will tie all of the crap I have into an informal pizza "history of sorts".
Just re-read some stuff while on hold with a company today. One of the first direct mentions of pizza by the slice I found:
“When pizza is eaten, it generally fills that momentary urge for a quick snack. In this sense pizza is symbolic of the present Atomic and Aspirin Age, in which the necessity has arisen for conducting business on the run and the hustle and bustle of everyday living have caught up with the working man, hurrying home to the suburbs. A tasty triangle of pizza won’t spoil supper and it may avoid for the train traveler those uncomfortable gnawings in the stomach. All this was undreamed of by our first business-wise Italian immigrant of 50 years ago.” --6/14/1956