I'm starting fairly gently with Diana Pullein-Thompson's Horses at Home/Friends Must Part (1954). This is an unusual book, made up of two novellas. The original dustjacket, by Sheila Rose, shows the lower leg position I was taught as a child, and which I have spent the rest of my riding life fighting.
You must just be able to see the tip of your toe, I was taught. Grip with your knees. You must be able to keep a penny between your knees and the saddle. I was taught to ride in the late 1960s, and even then this style was really more extant in the show ring than anywhere else. The Pullein-Thompsons had been teaching another way since the 1940s. It must have hurt to have seen some of the cover images that appeared on their books, but it's a rare author who has any control over their cover images.