I started on an old cookbook quest when the White House Cookbook was reprinted in the late 60's. I made a hot chocolate recipe exactly as listed - only to discover that the ingredients had become more concentrated over time. It was still an excellent, if thick hot chocolate drink.
There are a few YT channels that look at recipes and how they have changed over time, especially as artificial leavening was created and started being used.
Do you have a Half Price Book store in your area? They always have great old cookbooks. I think they may have an online store too but I like to actually browse through the cookbooks.