For me it was the Better Homes and Gardens New Cook Book. For example, the section on cream soups is a detailed explanation of the process of making any cream

For me it was the Better Homes and Gardens New Cook Book. For example, the section on cream soups is a detailed explanation of the process of making any cream soup, with a table of details and variations for each vegetable. I assume they didn't test every variation, but the first part obviously showed enough first hand experience to teach understanding not just rote repetition.