There are sources on the internet, but each one touches only one or two areas and some contradict each other. For example they say "two parts onions for other veggies" but videos always show equal parts onion to carrot and celery ratio.
I believe that simply explaining the process behind making a flavor base (how the veggies interact with each other, the role of fat, how dicing size affects the end result etc) will help us all make better informed decisions for our own preferences in whatever flavor base we choose to make.