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.

The Joy of Cooking still takes up the bulk of its pages with recipes. Though pretty much all cookbooks did pre-Web. You got recipes out of big cookbook volumes (as well as newspapers etc.)

I agree with your basic point though. And there are tons of cookbooks today that do spend pages on relevant techniques--though certainly some are celebrity chef memoirs interspersed with recipes and high-quality photoss.